Autofocus jumps around and often will not settle on the correct setting

Set to 'one shot' auto focus and try again

This sounds like, either you do not know how to use the camera, or the settings are set wrong.
As I have advised in many of my posts, set all menus to default or as much as you think the factory settings are. Then set them to what you like.
Also read your user manual. If you do not have one, download a copy from the Internet to your hard drive. View it with a pdf viewer, as most come that way.
Menu settings can affect many other settings, that is why reading and referring to the manual is a good idea.

The lock that's around the 4-way button that moves the AF point will stop it from moving. The AF switch (says C,S,M) on the body below the lens release button gets knocked to manual and turns off the AF

Try you 18-200 lens on another Nikon DSLR. If it still does not Autofocus. It is the lens. Sometimes the electrical contact points of the lens are not making correct contact with the camera. ok?

Hello,
It's possible that the reason the "pictures turn out fairly good" is that your f-stop is set high enough that even though the image isn't properly focused, you have enough depth-of-field to bring your subject into "near focus". Does your lens try to focus back and forth when you half-press the shutter? If not, double check that the lens is in autofocus and not manual focus. You can also try cleaning the contacts between the lens and camera. Take the lens off, and then one-at-a-time clean the contacts on both the lens and the camera, using a clean pencil eraser. Gently rub the contacts. Be sure to hold the item upside down so that the eraser debris falls on to the table, not into the camera body or lens!

If none of this helps the autofocus may have become damaged and you need to have the lens and camera looked at by Canon Repair.

Are both the lens and camera set to autofocus?

I had this problem awhile ago - you must have accidently adjusted the focal area - it is sooooo easy to do without realising it. Here is what I was told:
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Most likely, you have changed the AF point, which only takes effect in the creative modes (Manual, Av, Tv, P, etc)

There should be a button on your camera that has a grid like pattern above it, like
-
- - - - -
-

or so. press that, look through the viewfinder, and move the shutter or rear wheel, you should see the different AF points like up in red, and as you turn different ones will light up, and eventually all of them. also, you can look on the screen on top (or back, if its a rebel) and it will show you which ones are selected while in that mode, instead of looking through the viewfinder.

The RC-1 should be compatible with the Rebel XT. It can be set to a 2-second delay or no delay; pressing the button will activate metering, autofocus, and the shutter all at once.

First try another lens on the body and see if the problem is the lens or the body,if its the body the autofocus mirror could be out of adjustment, if its the lens, it sounds like the lens took an impact and needs a pro repair. you should be able to set body to manual focus and focus lens to infinty.

have you already pushed the focus point button - top right, under your thumb and then push the set button? Pushing the set button will flip between only the centre focus point and all 7.