Before attempting to clean or lube things it would be good to attempt to isolate the problem. For example:
- what is the actual symptom, for ex does the shutter stick open and the aperture stay at the taking aperture, or does the shutter close again?
- can you tell whether it's the body or the lens that is causing the issue, by trying a different lens.
- was a meter prism used in auto exposure mode? If so does it work reliably in manual?
- is the 6V battery in the body absolutely fresh? The drain on this battery is highest when the shutter is open during exposure, so a long exposure might temporarily drain a weak battery.
- clean the electrical contacts between lens and body (perhaps not the problem, but easy).
You can test the shutter repeatedly using multi exposure mode and/or the roll of backing paper without film.