M2s are like model Ms with some extra electrical gremlins (bad caps), given your symptoms i would either expect broken rivets (20+years old plastics rivets will break) or a damaged membrane (maybe from liquid spilled on it), usually, from what I've read bad caps will make it not function at all.
I would start my checking for broken rivets on the back of the membrane assembly, next caps are cheap and easy to replace so if you do not find broken rivets near the non working keys i would change them, and then it would be time to disable the sandwich to check the membrane. or wait for fohat to weigh in, he seems to be the expert of model M around here