Not the news you want to hear I'm afraid. I tried my NTSC tape, and guess what. I get exactly the same result as yourself. Get audio, and I get video for a split second. Darn darn darn!

I keep trawling though and post back if I discover a solution.

PS. Even longer shot, but if you have any North American friends with a AV IN enabled Mini DV Camcorder. You could get them to transfer your NTSC Tapes to miniDV and then use your Sony camcorder to transfer to PC. (I know, I'm getting silly am I not?)
:(