hoffbill,
I have two M-100s. One had a jaming problem in one which I corrected myself. Cleaning is important, but M-100s are old and sometimes the springs weaken. You can do this yourself or take it to a gunsmith. Change the extractor spring to a slightly stronger spring. This is the spring on the claw that pulls the case from the chamber. For the action to function correctly, the extractor must hold the case firmly until the bolt moves all the way back and the ejector kicks the case out of the action. If the case is held too loose or slips from the extractor before bolt moves fully to the rear the ejector will not kick it out of the action.
Be careful removing this part as it is very small and will fly out when the retaining pin is removed from the bolt. Wrap the bolt in a white cloth when driving out the pin. I did this on both of my M-100s and have never had another jam. I have a supply of small gunsmith springs and just cut one of the same diameter slightly longer, maybe a half to three quarters of a turn longer.