That's pretty much the set-up I'm using at the moment - Abelton & Wavelab. Yeh like Ze Mig said, check what sample rate Ableton is using as I don't have any problems like that.

For setting up loops in Wavelab, I left click to drag over the area you wanna loop, then right click and choose the little green markers - should be called "create new loop marker" or something like that.

The markers will then be either side of your highlighted area, there is a little loop option on the top taskbar (green too, I think) - just click this and it should all loop for you.

Oh, also - are you saving your wave file in Wavelab or rendering it? If you are rendering it, make sure you aren't using any of the degredation options in Wavelab, as that could be causing your sound quality probs.

Good luck