No great expert, but surely the Monitor takes its frames from the material it's displaying . . . . so why would it attempt to show 30fps rather than 50 (although my own Vegas 10 converts 50i to 25 fps, presumably it does this correctly?)
The issue of "full HD" is really one for the Software. Several well-known Programs have a new-version to accept HD, that is necessary so they can accept more "stuff", so the software needs to expand the areas of storage so it can keep tabs on the larger files. Simple enough in itself and surprising customers are required to pay so much again, just for a small addressing issue. . . . . but that's Commerce and we each are squeezed from all directions.
Certainly, rendering HD will put a considerable strain on a low-spec PC. There are stories of "lost frames" but suspect this is possible via a slow USB port, but "lack of spec" inside the PC only results in very lengthy Render-times.
I note that this Camcorder ( Sony HDR-S10 ) has a 40G HDD and this (with the latest software) "might" be the siource of the problem. If the footage is smooth on the LCD, then it should not be much worse when viewed on a TV - can you do this, direct off the HDD? . . . . .this "might" point to the PMB software . . . Noiseboy raises the issue of PC-Spec . . . . we'd love to know.