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The paper rollers COULD be dirty....and when they are dirty they dont push the paper as well as it should, which would explain why the paper is not fully exiting the machine, tripping the JAM sensor.
Have you tried to clean ALL the paper feed rollers.....not just the ones that grab the paper at the beginning......theres a few sets of rollers.
I know you said it was NOT a feed issue......and your right....its an exiting issue.....the exit rollers might be dusty, which would cause it to slip.......and again i know you said it wasnt this......but im just checking to make sure your aware of ALL the rollers in the machine.
Can you tell me which ERROR message you are getting for the paper Jam......the code will tell which area the jam has occured.
its P001 through P007