motor question

got my motor back put timing light on it the mark on the balancer is jumping so much i can't set the timing what could be wrong? i have a cloyes 2 piece cover on it, msd 8552 dist. never had this problem before i had it rebuilt any suggestions on what i may have done wrong? thanks

Cam walk.
Check it at about 3000 rpm and see if it stops moving.

Cam walk, loose outer ring on balancer. Sometimes a cheap neon timing light works better on an MSD system.