Strange Idle issue, since replacing ALT belt

[T?@\7]

I got my alternator belt replaced about 2 weeks ago, and ever since than, I have an idle issue. Sometimes.
Sometimes, once ive started my truck it'll idle normally around 1k +/- cold, and then after putting it in gear and backing out of the garage she'll jump up to 1500 and stay there anytime at idle. It doesnt happen all the time, maybe about 50% of the time. Anybody ever seen this? It seems like it has something to do with going in and or out of gear, as it will idle normally until I put it in gear.

 

[SKIDPLATE]

Have you done just the basic "look over the last thing that was done"? If you say that the problem started then, inspect the whole area where the work was done. Either something was left off, or something was damaged during replacement, or something was probably put on the wrong plug. If in doubt then compare your engine compartment wiring to someone elses. Hell, it could be something dumb like the belt is too tight or too loose or the wrong size so now a wire doesn't reach. Happy hunting.

 

[T?@\7]

ill look at stuff, though im a donkey when it comes to motors.
Still though, of any of your listed possible solutions, why would it only happen some of the time and not everytime the motor is running. For instance, I drove the truck twice tonight and the first time it ran normally (around 1k) and the second time, I got to the store and when I left it started running at 1500. thanks for your input