I tried capturing VHS a few years ago (not with PD but anyway).. There might be several reasons for dropping frames. Some ideas might be:
* Your computer isn't fast enough.
* Are you encoding the capture in realtime using a codec that's too much for your computer to handle?
* You've got a bad source. An old VHS-tape could prove difficult to capture depending on the quality of the hardware of your capture-device.
My hint is, for capturing VHS-stuff, is to use a DVD-recorder actually, and then transfer it to the computer. Some DVD-recorders (Pioneer comes to mind) have a TBC (time base corrector) that does wonders with old VHS-recordings. It saved me alot of trouble anyway.