91 Trooper timing problems

I have read all the posts on this I can find and I have found some great advice but before I tear into this thing one miore dumb question. I have a 91 four cyl after trying to start it up in zero degrees cold it always starts fine but after a few days of this it seems each day it ran a little worse each day like the timing had moved a bit each time I started it till now it will hardly run I think it jumped or slipped time. fireing at TDC but poping back out the intake a bit belt is tight and looks good I am in prosses of takeing it apart but is it common for the cam to slip on these things? just that the belt is tight and looks very good but you never know. any advice would be fantastic