I have a 1994 f150 that has sat for a good portion of its life. One of the first things that I had to do was replace the headlight switch because it was shorting out. The gas gauge only reads the first 12 gallons of the tank which isnt a big deal but now Ive noticed that the water temp and oil pressure are moving around more than they should. mostly they both raise when I roll up the windows or turn on the lights. They also climb when I accelerate and then return to a little above where they were before accelerating. Is there a constant voltage regulator on these trucks? or any ideas why its messing up?

 

I would check several things. In order of ease and cheap to test:
1) pull the battery and take it to your local parts store to have it tested.
2) check voltage when running. Put a DVM on the battery leads and have a friend vary the rpm's. Do the same with headlights on. The readings at low rpm's will show the headlight on value to be lower, but at higher rpm's, like a 1,500 rpm, the readings should be the same and at least in the mid 13 volt range.
3) if all that looks good, start looking for poor electrical connections. Most highly suspect would be the ground connections to the cab and core support.

I'm sure other will have other suggestions as to where to look.