Sometime ago I aquired a Nikon F5, which suddenly is showing some odd behaviour. When I load film, it advances as usual to frame 1. But when I push the trigger to take one frame, it takes the frame and keeps up the mirror. At the same time ERR message shows in display and the red LED over 2 filmtransport is flashing. Pushing the trigger again, does nothing. But when I momentarily push the main switch to the light position and after that push the trigger again, the mirror comes back down and the film advances to the next frame. This continues throughout the film. Sometimes resetting the camera with the light switch doesn't work. ERR message keeps staying in display and the camera is very unresponsive. The only way I can have things going again, is to release the backdoor and close it again. Pushing the trigger after that of course again gives a film start advance sequence and a 1 at the counter. Any ideas to put the camera into normal function mode are greatly appreciated.