I've got a good 52" LCD - Samsung 850 series. Blu-Ray discs play perfectly through our PS3. HD regular programming via Comcast looks great.
HOwever, when we watch an On Demand HD movie, the picture suffers from lag/blur. At times, even the audio track slips out of sync. Is this a Comcast quality issue? Or is it some setting I may be overlooking in my Samsung?
-wg

It's a Comcast issue. You've proved that by saying that your regular Comcast HD looks great. Because your Comcast HD signals for both regular and On Demand programming come from the same box your Samsung doesn't know the difference. That is, of course, unless you are splitting off the coax through a splitter and are watching regular HD programming via the QAM tuner in your TV.

On Demand requires a stronger, more pure and undiluted signal than regular cable channels, whether HD or not HD. I have had similar problems. Removing an extra cable in my line, with its two connectors, solved the problem. Check all your cables and connectors, and try for as few breaks in the line as possible. One long uninterrupted cable will give you a better signal that several shorter ones. This might clear up your problem.