I'm quite baffled and looking for some help.

I have a TV Excel (some cheap TV Tuner/Capture card in Australia) and actually get excellent results. It works with almost every capturing software out there. I use Cineplayer DVR for a quick and dirty capture, and VirtualDub for great clean and ready to edit results (using MJPEG @ Quality 19. Now the issue.

I Connected my VCR to the capture card and leave the settings on Composite, thus anything i play through the vcr, like TV or Video, can be captured. If i'm watching TV and start a capture, i select 25fps and i will capture at 25 fps and in a 30minute capture, will drop only like 8 frames, but if i press play on a VHS tape, suddenly the frame rate will drop to around 15fps.
Some tapes would still capture for me at 25fps per second and i have only just worked out that a possible explaination is that VHS tapes that capture @ 25fps are VHS tapes i taped off TV. But ORIGINAL VHS movies will instantly drop to 15fps.

Is anyone able to explain why this happen and if there is some way around this problem. Since it all goes through composite, i wouldn't think the capture card would understand the difference in signal, and if it could, why it would make such a difference?

Thanks.
Other Specs. P4, 512meg, 2 x IDE 7200rpm drives. SB live, 64meg MX400 Video, Windows ME.