Here's my set up, just to get it out of the way.
Pioneer PDP-4320 Plasma Display with the External Media Receiver
Motorola DCT3416 I HD-DVR Cable Box from Comcast
6' HDMI Cable
I recently upgraded from component video cables to HDMI cables for both sound and video. After doing the upgrade I developed an issue where when switching the cable box from an HD channel to a SD channel (or the OnDemand menu from Comcast) the picture drops out completely. This happens regardless of the direction of the switch, SD -> HD or HD -> SD, and the only way to fix the issue is to power cycle the Plasma TV or swap to another input source and then back to the cable box's source. When the switch occurs there's a several second delay while the TV reacquires the signal before displaying it properly. When the picture drops following a switch there's usually a couple second display of white noise/static before the TV goes dark.
I found a similar thread in the forum (sorry, not allowed to post links yet) discussing what sounds like a very similar issue between a Sharp Aquos and a Verizon FiOS box. The "fix" posted there doesn't seem to work on my box.
Can anyone help a guy out here?