When I picked up my Mamiya this morning the lightmeter in the prism seemed to have gone bananas for some reason. In whatever direction I point the lens, it tells me the measured shutterspeed is too high (OVER blinks in the view finder).
Happens for all aperatures, all times, all ISO values, and both SP and AV modes.

I shot a few frames, and suddenly it started to work. So I thought it was only some temporary problem, and went on with my business.

Now (aprox 6 hours later) the prism has started to act all crazy again.

I've run a battery check and it says that the battery is OK.

Any ideas?