I don't think you're after a line doubler. Any interlaced low scan signal coming into your PC is automatically line doubled and converted to progressive when displayed on the VGA port.
The quality that this is done at varies on the video card and/or your software.
If you have a fast enough PC and a compatible capture card I'd take a look at dscaler, do a search on the net. It's well known as the best deinterlacer/inverse telecine software around, and it's free. Should clear up your jerky playback nicely.
Cheers,
Arkay,.
Edit: Spelled Dscaler as descaler... But was talking about Dscaler