Flash video playback problems

new to windows/pcs. (apple guy for ages). bought stock(no upgrades)inspiron 546 to play internet video 1 month ago to my 32" hdtv through dvi cable. Using fiefox(though problem same with explorer). comcast cable internet. 1st three weeks it worked fine-hulu/cbs/fox no problems-as good a picture as i could expect-seamless playback. 

Last week sometime between wed and fri it stopped working right. there were no updates(on manual update). it will play fine on small screen, but not whole screen-it jitters-like fast slideshow.Only flashvideo-silverlight works fine, netflix works fine.  our other dell laptop(not mine) plays same video same connection to same screen(svideo connector) with no problems.  i did a speed test on internet connection and got a supposed rate of 30mb/s...

went and did all updates, no change. reinstalled everything from scratch w/all auto updates and new flash player, no change. last night Tech support from dell spent 2 hours doing updates, no change, and no resolution(i had to have dinner).

anyone have any ideas? the cpu is maxing out during video playback...should it be?Plays back flash video not much better than my 10 year old 500mhz g4.

Thanks in advance for any input, Ben.