5D freezes in a short time after powering on

Hi there,

I'm having an issue with my 5D Classic: the camera powers on and 'functions' for about 20 seconds, but then it freezes, leading me to switch off the camera and re-insert the battery for it to power on again. Sometimes it functions for ~20 seconds, sometimes it does not power on at all, and still at other times it functions for a few seconds before freezing. When I try to take a photo, the camera inc. the lens freezes every time; the shutter shuts, gets stuck, and does not open again until I re-insert the battery (then it 'snaps' open).

Any leads as to what the issue might be here?

I appreciate your input.