Hi Steve and welcome
As you've read, the problem I (and one or two others) faced was not common to all and may ultimately been down to individual settings.. eg, terrain vertex. My fix was done by a clever chap who looked at the flattens and found several holes. He then set about 'filling' the holes and this did the trick for me. The original was fine on his system may I add.
I think it's probably true to say that GenX itself may have been partly to blame as it seems to install (or can install) slightly differently on each system depending on what is underneath. I started with GenX v1 then 'upgraded' to v2 with the update discs. I think DaveG has already commented that this update
didn't actually bring v1 up to full v2 spec (even though we all presumed it did) so adding the v3 updates to a
not so full v2 may have caused the terrain level differences or certainly, not helped.
I can't remember which Hamble files were actually changed but there were a few in order for it to display correctly. I'm happy to upload the modded version I have if you like Steve
EDIT: Here's the modded version I'm using Steve. Remove your original (but keep it safe just in case) and try this one..
HambleMod
ATB
DaveB
