Mamiya 645 ProTL query – help from anyone out there?

I have a Mamiya 645 ProTL with the prism viewfinder and the motor-drive.  There are two battery power sources: one small 6.5V cell in the main body, and 6AA cells in the motor-drive.

The meter and the shutter are, as I understand it, operated from the small 6.5V cell, whilst the AA cells are just for the motor-drive.  I almost always take multiple spot readings with my Sekonic meter, relying only rarely on the camera’s meter (basically, when I think the AV measurement will be sufficient – I’ve found the spot meter to be slightly off, and I trust my Sekonic meter more), but last week the camera meter just stopped working altogether.  This has happened a couple of times before, and I’m beginning to wonder if there’s some kind of reason for/pattern to this, and if others have experienced the same thing.

I replaced the 6.5V cell (I had a new spare in the camera bag) and that had no effect.  I then, despite being fairly sure this would have no effect, also replaced the AA cells with new ones – I was right, this didn’t make it work either.  Strangely, the shutter seemed to work just fine, and the battery check light for all combinations of battery was also ok.

When I came home, I tested all the batteries with a multimeter and they were all fine (so I put the old ones back in!).  I took the prism viewfinder off the camera body and there was no debris or dirt visible that would impact on the electrical contacts.  When I reattached it and tried the meter again, it flickered, and then worked.  I thought this meant a loose contact between the prism meter and the camera body but have checked this multiple times and it is all really firmly held together, so I don’t think it is dirt/loose contacts between the body and the viewfinder.  If there is internal damage to the contacts, I’ll have a hard time fixing that, but if there’s another explanation, I’d like to know more.

Anyone have any ideas why this might be happening?  If you know anything about this – particularly how to avoid it happening! – do please share, either in the comments, or by email here.

Thank you very much for your help!