Timing problems

before you rip it apart a few things to look at

check the vac line to dist (Or is it full centrifical?) to make sure u dont have it plugged in to full vacuum at idle.
Check harmonic balancer to see if it has spun the center on it

how are you reading the timing ...with an advance type light or a regular lite and a degreed balancer
since you didnt mention it changing every time you check it I would assume the gear hasnt spun on the dist.
when it breaks up above 2500 whats the timing marks doing ?
throw a vac guage on the motor and see what is happening internally is vac steady ? low? wavering severly back and forth ?
no easy way to tell 100% for sure if chain jumped just one tooth

all i can think of at the moment