This is not really what you asked for, but I had a similar problem, the mirror sometimes jumped twice, and sometimes it got stuck halfway. It always resolved if I pushed the button again, but at one ocation I thougt the mirror was done for good.

Worth to mention is that the camera had trouble saving pictures and viewing pictures, sometimes the screen just turned itself off.

In the end it turned out to be the fault of the battery. When I got back home to charge the battery, it was dead. It was a third party battery, from Phottix. It was designed for Nikon D200, but the chip was faulty from the start, so it just worked on D70(s) and similar models.

From that day, I only used the original battery I had left, and the problems have never occured again.

Maybe just a bad copy.

Also you will get an 'err' message if you have an AF lens which is not locked on the smallest f-stop, at least that's what it does on the D50. Don't ask me how I know this.

Also, what specifically does a 'cha' error message mean? I believe it refers to an inappropriate CF memory card but there seems to be an epidemic and there are quite a few D70s out there being sold as 'parts or repair only' showing a 'cha' error message.