My issues with this game doesn't seem to end. Everything was perfect until suddenly the game started to slow down and even freeze at the moments I get pickups or get hit by enemies. Outside of those moment, performance is flawless. It used to be flawless trought the whole game until now. Just now, weird lines started to show also when I get pickups or get shot. Those moments seem to create problems.

try using eduke32
http://www.eduke32.com/downloads

Thanks but this happens WITH Eduke. Sorry I forgot to mention that.

how does vanilla run?

Try forcing OpenGL rendering. To do this, add -forcegl to the end of your shortcut. It should then look something like this:
"C:\Program Files\GOG.com\Duke Nukem 3D\eduke32.exe" -forcegl

Vanilla's totally cool. But I can't play half of the game again! I progressed a lot with the eduke32 one.
That made it even worse. Now the visual artifacts(weird lines) are also seen in the main menu :(

What sort of video card do you have?? I played vanilla Duke 3d just fine on an Intel integrated graphics card....with updated drivers :D

Yes I'm in the same position. Vanilla runs fine with my on-board intel graphics chip. I have a sony vaio laptop.

Thanks a lot. That solved the problem. For good I hope.

I have a win7 64bit, dual core intel processor, 485gtx, 4gbs ram, and the game is unplayable with eduke32 on the lowest settings.

Any advice?