I had the same issue when I ran my 94 at the track the first time to get some base numbers. I would set the timing and make a run, then get back to find the timing all off by like +/-5*. I had brougt a new cap and rotor with me and swaped that in at that time and reset the timing to 14-15* and it stayed there. Also just with this new motor I had the issue again but with a new cap, rotor, and harm. ball. It ended up that the dizzy hold down bolt is just a pain the arse after full removal and would somehow wedge to get a "tight" feel but let the dizzy turn after shutoff and re-start (to check the timing after a restart I found is a quick way to check before un-hookeing the light/gun). I had to turn the bolt and wiggle the hold down and after a lot of turning and flipping of the 1/2 wrench got it and it stayed set. The dizzy even stayed somewhat tight when th bolt was loose my guess is from the wedge action of the bolt/hold down until it was moved by the engine. I redid the bolt alittle the first time and then grabbed the dizzy by hand real hard and twisted it loose (it took some force but not like trq. a head bolt), thats when I was like Ahh Haa.
