I have a 1988 pickup with the 3.0 3vze. I just replaced the timing belt, water pump, etc. It ran perfectly for a couple days, then all of the sudden it jumped timing and was barely running, so I tore it back apart & reset the timing again. Once again it ran perfectly for the rest of the day, only for the timing to jump again. I turned the crank over twice and everything was lining up fine, arrows facing out on the belt, new tensioner and everything. I even used torque specs. Any ideas on what may be causing this?

Spring tensioner btw

 

you can easily skip the timing belt if tightening the crankshaft bolt incorrectly, like I did at first, but if not that, then your belt could be to loose> Be sure before tightening your tensioner bolt, force tension on it with a crowbar before tightening it to get it real tight, I bet that is the culprit, tighten that tension before you lock the bolt down, don't just rely on the spring.