Out of curiosity, what is your source material? If you are capturing from the same starting point on the tape, its possible time-base errors on the tape are causing the capture device to stop.... although you likely tried other tapes. For SD video work I would lean towards a modern machine, they can all do MPEG-2 software compression in real time or faster.

I usually lean towards lossless compression (1 hour of 720x480/60i HuffYUV compressed video is roughly 35-40GB) and do a final pass to MPEG-2 or h.264 post-edit depending on the destination media (DVD, Youtube, etc).