Video not smooth on Vista Media Center recordings

[RA56]

I recently bought an HP Pavilion Media Center TV a1777c Desktop PC running Vista Home Premium. It has an Intel Core 2 Duo E4300 (C) DC 1.8 GHz processor and I upgraded it from 2 GB of DDR2 PC2-4200 memory to 4 GB of DDR2 PC2-5300 memory. It has a Hitachi 300 GB SATA 3G (3.0 Gb/sec) hard drive running at 7200 rpm. The video card is a GeForce 7500LE with 256 MB DDR memory and an unnamed TV tuner card with FM tuner.

When I watch a recorded a TV show (from analog cable) recorded at the best quality, the video playback is jerky at times. I am assuming that the processor, memory and hard drive specs are more than adequate for a Media Center PC and I suspect the problem is caused by either the TV tuner card or the video card. I wouldn't mind upgrading if I could determine which component is responsible for the playback problems (i.e. Is the problem caused during recording or during playback?).

[edDV]

Recording vs playback:

Does it happen to the same frames each time you play it?
Also, do the glitches happen in high motion clips only?
What is the CPU load when recording a show? *

(see task manager > ctl alt delete - task mgr - performance)