I'll take a swing at this one, if no one else wants to.
I checked the specs on your Hercules card. It looks like it has hardware decoder ICs, but not encoder IC's. That may mean the actual video capture and compression is being done with PC software only. With the larger format, your PC may not be fast enough to both compress the video and display it. I know when I first started out, I used a Hauppauge WinTV PCI card and on my 1.4gHz Athlon, I couldn't go larger than 400x300 without losing frames unless I selected a low compression codec, like HUFFYUV or the Brooktree codec. With DivX, mpeg1/2 or XviD, 400x300 was all I could manage.
Later I got the WinTV PVR250. It has a hardware mpeg2 compressor IC that lets me do 720x576 with only 45% CPU loading. Most of that load is due to the software decoding needed for displaying the video while recording.
I can only suggest you try to capture with a low compression codec, like the Panasonic DV codec. DV compression is pretty simple and fast, though there is always a little distortion, since it's a lossy compressor. The resulting file will be large, on the order of 4 gigabytes for 15 minutes of video, but if your hard drive is formatted in NTSF instead of FAT32, it shouldn't be a problem.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70