I have a 645 Pro with the prism meter. Forgetting the meter, have you tried shooting based upon your handheld meter and changing shutter and aperture? I'm trying to ascertain whether the problem is with the camera or with the meter. If the camera exposes correctly based upon your handheld meter, than you know the meter is the problem and can be replaced pretty cheaply. If your exposures are all off (and you've verified your light readings) than you have a shutter or aperture actuation issue which can be more expensive. I'm presuming that your processing is standardized across both 35mm and M/F platforms.