Following an advice I found at another forum, I changed the CMOS battery today and to my surprise things got better.
Now the screen freezes occur between longer time intervals (every 3 minutes), and they last for only 2-4 seconds.
"Dmesg | tail" shows the same 2 lines everytime this happens:
[ 1072.401066] NVRM: Xid (0000:02:00): 8, Channel 00000003
[ 1074.398747] NVRM: os_schedule: Attempted to yield the CPU while in atomic or interrupt context
I hope this helps a bit...