Tach start jumping when i reach 65 to 75 milles per hr

If this is a points type ignition, check for excess play side to side in the distributor shaft, and also for timing chain slack--easily tested by removing dist. cap and rotating engine back and forth with wrench on crank pulley bolt. If rotor doesn't move immediately as crank turns you have loose timing chain.

I think you are going to need a transmission rebuild, sorry for the bad news.

You need to have the tach replaced or take it out and have an auto repair shop or the dealer send it out for repairs, that is if it's repairable.

Here the good thing and this it will make it easy, The reason for that jumping tach meter tells you that there is spark and no fuel. Without fuel the spark KV voltage is 30kv. and it report to the tach. So you got spark and no fuel. If you had fuel the tach will show during cranking little tiny,tiny movement that you wont even see. So do this turn the key in the run position cycle it do you hear a humming noise from the rear you should that is the pump, they are sort of loud, If you dont you'll need a fuel pump. Even if you heard the fuel pump the fuel delivery may be low of pressure to start the engine. by the way try this have some one crank it and you hit the bottom of the tank may be you can shock it back.

When some error codes are displayed the computer puts the car into limp mode - this restricts the speed of the car so you can get home or to a repair shop - you should not drive it now until the problem is solved to avoid any further damage.
You may just have a faulty camshaft position sensor on one bank or maybe the timing belt may have jumped a cog and require replacement.
Code readers are very reliable - the code readers only displays the fault information that is sent from the computer - this tells the mechanic the exact fault or gives them a good starting point to diagnose the problem.
Sometimes resetting the fault codes in the computer with the code reader will fix the problem (it may have just been an incorrect fault indication) - if not - it will default again which confirms the fault and you will need to fix the problem.

I would suspect the rear axle speed sensor.

well my friend it sounds like a clear case of your timing jumping out of mark. (jumped a tooth). Youll need a complete timing service and I'm sure you'll be good to go again. If Im wrong, I'll eat my shoe.

It sounds like the engine "backfired" and blew the cover off. This and having the tach "jumping" means that the engine timing is erratic. This can be caused by the crankshaft position sensor or the ignition module. It could also be cross firing at the coil pack,if so equipped. Make sure all of the wiring to these components is good.

This is a classic symptom of a defective driver in the tach's circuit board, it can be repaired by a authorized GM electronics repair center.