Sorry, between email support and some real life stuff taking up a bunch of time the past couple weeks, I have not been checking the forum very often, except to delete spam.
Do you get the same error leaving act 2 via the stairway near Samuel's lair? Most likely you would, if the problem is with the map itself.
Please email your save at the end of the act to firstname.lastname@example.org.
For the performance problem with items and tooltips:
Primarily with the disk version, a few people had general performance problem with Beyond Divinity, particularly (sometime exclusively) with the steam vent effect, such as on the initial level of the game. While not the same symptoms, you can try the troubleshooting suggestions and workarounds for that. See New machine...BD is in slow motion!
For the GOG version, here is an unofficial download for the 4 files in the 1.506 patch
(div.exe, DivDialogSystem.dll, Osiris.dll, slash1.dll). Extract it into the Beyond Divinity folder, overwriting the files there (make a backup first, if you are so inclined). The archive is in the 7-Zip
.7z format (rename to .exe if you don't have an archive manager that supports this format). See this GOG topic