I just want to share a possible solution for anyone having this weird refresh rate issue that causes the screen's upper area look like a broken glass. Kind of hard to describe it with my limited English vocabulary, sorry.
Anyway, the following helped me.

In cam_ext.cfg, change the line
;force_windowed
to
force_windowed
.

The game stayed in fullscreen for me though. No mods/patches except the GOG's applied.

By your description, that sounds a little like tearing; if the game offers the option (I don't know whether I've ever looked), have you tried enabling vsynch?

Thank you, that is indeed the proper term.
Vertical synchronization was enabled by default as far as I know. Also tried using different display refresh synchronization modes, and forced it via AMD Catalyst Control Center with no luck.

Ah, fair enough, and rather odd.

Well, in that case, thank you for posting your solution! It may well prove useful to someone else experiencing the same problem.