I had the same problem copying VHS and Hi-8 tapes. And I'm using an ATI 7500 AIW. Made no difference what capturing software I used, the longer the video was, the more the a/v would go out of sync. Turning off Smart Encoding helped for short videos (no more than 30 minutes), but substantially extended the processing time.
Since I am now doing my home video recording with a DV camcorder, I connected the VCR/Hi-8 camera to the inputs of the DV and then output to my computer. The DV camcorder does all the digital conversion and, voila! No problems.
I know this doesn't solve your problem at hand, but if you're going to go with DV, wait on doing those transfers and use the DV to do it. I can't tell you how many hours I wasted before.