1st: plz excuse my english

I have a serious problem when capturing from VHS (PAL). The amount of dropped frames seems to depend on the TV station I've recorded the tape from. When I capture directly from the same station (NOT from a VHS copy of the station) it works fine. Half of the dropped frames are always exactly at the same position, the other half varies. These "fixed dropped frames" are clearly visible on the computer screen while capturing but on TV the tapes look fine, absolutely no dropouts are visible there.
Usually I can capture a long time without loosing more than a frame every two minutes. On the problem stations I loose around 30 frames per minute. So I think it has nothing to do with my hardware. I don't think it has something to do with audio, too. The frames are also dropped if I don't capture any audio at all.