I'm pretty sure it's something to do with your multidec plugin or it's setup, since the symptoms you describe are pretty much like watching live tv on a channel that wasnt decrypted.
I'm not really sure, but another guy I know had stuttering on his PC when he tried the exact same multidec plugins as I was using - everything else was exactly the same (same capture device, cable network etc). We put it down to the differences in the speed of our PC - he was using a XP1800+, I'd tried tried the same setup with a P4 2.6Ghz, a Pentium Mobile 2.0Ghz and an AMD dual core 4400+. Luckily in his case we were able correct his issue switching to a different multidec plugin.
I dont think multidec wrapper layer I use is the highest performance (not written by me), but seems to work well for most, but you may find your CPU requirements are higher than you might get using the same multidec plugin with some other app.
You could see if disabling the multidec support, and viewing a unencrypted channel has the same behaviour. I suspect it'll be smoother.