I have a Nikon FM2n which seems to work fine and really smoothly, until I move the speed dial onto 1/30 sec. It then lets me take the shot, but the shutters remain open and the mirror is locked up and I can't wind the film on. The only way to unjam it is to move the shutter speed dial to 1/500. Then the mirror drops and the curtains close. It seems to work perfectly on all other speeds.
Edit: Actually the same happens on 1/4 and I have to move the speed dial to unjam it. This mainly unjams at 1/60, but sometimes I need to turn the speed dial to other speeds to unjam it.
Sounds like the slow speed governor is dry, a simple enough fix (if that is indeed the problem) with a CLA.
But if you just bought the thing and it's doing this, return it.