When I shift to thrid gear the bike lunges forward like it is skipping a tooth on a sprocket. What causes this?

36 tooth
36 / 13 = 2.77 ratio
25 / 9 = 2.77 ratio

I would lube everything and check the derailleur adjustments and cable tension. Some Campy chains are 'notchy/clunky' all the time. It's just the way they are. If the bike is low mileage most likely nothing major is wrong. Check the derailleur hanger too just in case you dropped the bike and bent it or something. The shifter 'G' springs will wear out before the derailleur. When the springs are worn, the shifts will be imprecise and will jump from gear to gear without shifting. One thing with Campy stuff...it DOES take awhile to break in. Unless the shifting is horribly off, I wouldn't worry about it - just ride it more - and see what happens. Mirage is a good product, I think inline with Shimano 105 stuff.

You have conflicting info. If there are multiple gears then there is a derailer. You should take to a bike shop where they can look at it. It is probably a very small easy adjustment.

Look at the derailleur hanger alignment first to make sure its not all bent up, then the adjustments.

http://www.parktool.com/blog/repair-help/categories/derailleur-systems

http://sheldonbrown.com/chains.html

Use search engines to answer generic questions.

The shift drum would not make any noise, ever. The drum will outlast all of us. All it does is move clockwise and counterclockwise as the rider pushes the shift lever up or down as needed. Go to an empty parking lot or roadway and start out slowly in 1st gear, slowly gaining speed. Does the knock show up? Now go into 2nd gear and slowly gain speed. Is the knock there? Do the same for all the gears. If the knock is only in one or two gears the problem is a broken gear tooth. When the broken gear is replaced, also replace any other gear the broken gear meshes with. Whatever broke the tooth also put the same pressure on the meshing tooth. If the meshing gears are not replaced you will have the pleasure, ( and expense ), of doing the repair all over again when the cracked tooth on a meshing gear decides to break off. This usually happens within the first hour of riding the "fixed" bike. If the test shows no knock in the gears, then chances are the knock is a cam chain needing to be tightened. Please rate my answer. Thanks.

You can add teeth to the front sprocket or take teeth off of the rear sprocket. In the case of your bike , I do not think there is a 17 tooth front available so your best option would be to shorten your chain and drop from the 44 tooth rear sprocket to 40 tooth or a 36 tooth rear sprocket. That would be like adding one or two teeth to the front sprocket. Your stock is 16/44 so decrease from there. Be sure to ID your chain, get a chain breaker, and a new master link.

By the "gear arm" I assume you mean that part of the rear derailer that the chain zig-zags through. This arm moves in two ways. It moves in and out (toward the wheel and away) as you move the shift lever - which is how it changes gears, by moving the chain onto different sprockets (gears). It also moves forward and back (toward the front and back of the bike) under spring pressure. This forward and back motion is necessary to take up the slack in the chain when you are using the smaller sprockets (the smaller the sprocket, the "higher" the gear). When the wheel has been removed, this arm moves forward (under spring pressure) as far as it can to take up all of the slack in the chain. All you have to do is grasp this lever and pull it toward the back of the bike, allowing the lever and the chain to pass behind the sprockets (gears) as you pull the wheel back into it's position. You can safely pull this arm forward and back at any time without doing any damage. One more tip-- I find it a little easier if you first put the shift lever in a "middle" gear position. Then when you pull the wheel into place, you should aim to put the chain on a middle sprocket. You don't have to hit the exact correct sprocket as you can simply lift the rear wheel and turn the pedals forward (after installing the wheel) and the chain will move to the correct sprocket corresponding to the setting on the shift lever.
Good luck! Please vote if you found this helpful. Al K

How is your clutch? Does it slip or drag when starting out? When you put it into 1st gear when starting out does the bike lunge forward? Hold it in first gear with the clutch in and rev the bike up. See if the bike wants to move forward with the clutch in.It sounds to me like your clutch isn't set right or getting worn out. Much easier to shift at low rpms on a clutch than high rpms. I hope this helps...

For confirmation of a broken third gear tooth, drive slowly and shift up to third gear. Give the throttle a gentle rotation to slowly speed up. Listen for clunking sounds from the gearbox. If you hear it then you know there is a broken tooth. ALWAYS replace the gears in sets. If enough pressure was on a gear to break of a tooth then the same pressure was on the gear it was engaged into. Yes, only one of the two gears is missing a tooth but the other gear has a cracked tooth just ready to break as soon as you button up the engine after replacing only the toothless gear.

I am a bit removed from the cost of todays repairs so I suggest you give a call to your local dealer. If I had to guess, I would say about $600.00. Sorry for the bad news.

Top rating for this answer? Thanks!

With your rear derailleur shifted all the way to the smallest gear in the back, pull the chain forward over the crank ( the gear that the pedals are mounted to) from the top and stretch it around forward over the gear. Since it is used , you will have to see if you need a new chain. From the front position of the crank gear, about the 3 o'clock position, move a small section of the chain forward off the gear and see how much of the tooth in the center of that position is showing. You should not see the tip of the tooth . If you do it means that you should definitely replace the chain. On a 5 speed with the chain coming off that way , it means that either your rear derailleur is out of adjustment or your chain is worn out and is skipping to the point where it will come off. If your chain has come off at the rear derailleur and is stuck between the spokes and the gearset you will have to gently pry it out with a screwdriver or something suitable in a manner that will first of all not hurt you and then not damage any part of the bike. Bent spokes are hard to deal with and cause a bunch of other problems.I should have mentioned earlier to rub some hand cream or vaseline in to your hands to make the grease and dirt easier to wash off afterwards. Let me know and I will help you further if you feel comfortable and have a place and tools to work on this.