On my 1000s with the pd metering prism the main shutter speed dial should be set to the red position, where it is locked in place and use the shutter speed dial on the prism, I leave mine in that setting all the time and just use the prisim meter and shutter speed dial, My mirror always returns to rest after firing the shutter, never sticks up, I have just had a very quick look in the manual and the only thing I can find is mirror lock up, check that the mirror lock is set to normal, so in short when using the metering prism leave the main shutter dial in the red dot, use the meter dial in the prism, and make sure the mirror is not locked up, can't think of anything else