Videos now freeze/stutter on occasion?

[AlienAndy]

I have been using Media Portal for a couple of weeks or more now and it is superb!
But just in the last few days I have begun to notice a few second freeze now and then when playing pack videos ( avi ). Never had this problem before. Even the controller seems to become unresponsive at times? For instance, I was watching a Top Gear episode and a third of the way through it the picture froze but the sound carried on. Then after a good few seconds it started again. While it freezes I can't do anything on the remote either?

I know video codecs and such like are all properly installed. Besides which, they have all been working perfectly before. I have even checked to see if my hard drives ( 2 x 500Gb SATA ) needed defragmenting, and they don't. It seems though that the longer I leave my htpc on the less chance that I get this problem. Is there something that Media Portal does in the background while I am watching a video?

Any advice would be very much appreciated.

Thanks.