Sounds like memory is corrupt, or faulty.

I can suggest the following, in the following order:

1. Delete all stored addresses. On some older OS versions, too many stored addresses caused memory issues.

2. Load new OS. If the nav cannot read a CD, maybe CD laser is faulty. Of course, make sure the OS is in the correct Mode2Form1 CD format. If you use a Mode1 CD, you'll crash the nav
Of course, make sure you are loading 3-1/63 (the best and last OS ever made for the Mk3)

3. If that doesn't fix it, then you have faulty flash memory. Send to SiemensVDO in Germany for repair, or replace with a Mk4.