Hello everyone. Thanks for your replies.
The GPU is, indeed, an nVIDIA, GeForce GT 630.
What lines should I filter on syslog?
If I grep on nouveau, there are thousands of lines looking like
Oct 16 07:27:59 ivan-All-Series kernel: [ 3545.409232] nouveau 0000:01:00.0: gr: TRAP ch 2 [007fb01000 Xorg[1501]]
Oct 16 07:27:59 ivan-All-Series kernel: [ 3545.409243] nouveau 0000:01:00.0: gr: GPC0/TPC0/MP trap: global 00000000 [] warp 3d000d [OOR_REG]
Oct 16 07:27:59 ivan-All-Series kernel: [ 3545.426413] nouveau 0000:01:00.0: gr: TRAP ch 2 [007fb01000 Xorg[1501]]
Oct 16 07:27:59 ivan-All-Series kernel: [ 3545.426424] nouveau 0000:01:00.0: gr: GPC0/TPC0/MP trap: global 00000000 [] warp 3c000d [OOR_REG]
Oct 16 07:27:59 ivan-All-Series kernel: [ 3545.443718] nouveau 0000:01:00.0: gr: TRAP ch 2 [007fb01000 Xorg[1501]]
Oct 16 07:27:59 ivan-All-Series kernel: [ 3545.443729] nouveau 0000:01:00.0: gr: GPC0/TPC0/MP trap: global 00000000 [] warp 3d000d [OOR_REG]
and
Oct 16 23:58:30 ivan-All-Series kernel: [62976.327626] nouveau 0000:01:00.0: gr: TRAP ch 5 [007faf8000 systemd-logind[760]]
Oct 16 23:58:30 ivan-All-Series kernel: [62976.327631] nouveau 0000:01:00.0: gr: GPC0/TPC0/MP trap: global 00000004 [MULTIPLE_WARP_ERRORS] warp 2a0001 [STACK_ERROR]
Oct 16 23:58:30 ivan-All-Series kernel: [62976.327640] nouveau 0000:01:00.0: gr: TRAP ch 5 [007faf8000 systemd-logind[760]]
Oct 16 23:58:30 ivan-All-Series kernel: [62976.327645] nouveau 0000:01:00.0: gr: GPC0/TPC0/MP trap: global 00000004 [MULTIPLE_WARP_ERRORS] warp 1a0001 [STACK_ERROR]
Oct 16 23:58:30 ivan-All-Series kernel: [62976.327653] nouveau 0000:01:00.0: gr: TRAP ch 5 [007faf8000 systemd-logind[760]]
I had no similar problems on this PC before installing ZorinOS so I want to think RAM is OK. I am executing memtest tonight and I'll tell you about the result anyway.
Thanks all for your help.