They probably just need to be disassembled, cleaned, and properly re-lubricated.

Some parts may need replacement. (I had to have a spring replaced on one of the silver shutters once when it was having similar problems as you describe.) If this is the case, your technician should be able to get factory parts from Mamiya if you have a black-shuttered lens, but will need to pick the boneyard for parts if you have an older silver-shuttered lens.