I also hope that will fix it...
But I still can't seem to understand why it only happens in panny plasma.
To cover something on the plasma makes more sense like what sir lucky23 first mentioned.
Anyway, it won't hurt to try. 
Actually it doesn't happen only with Panasonic plasma. The ones having problems in the WDTV forum have various LCD TVs and varying brands too. In most cases, their problem has to do with their TV's energy saver feature, or light sensor feature, such as the C.A.T.S. (Contrast Automatic Tracking System) on some Panasonic plasma TVs:
"Some TVs and other electronic devices leak or emit IR that can interfere with the WDTV's ability to receive signals from the remote. The WDTV's IR receiver is located between the two LEDs on the front of the unit. The small plastic oval in the front center of the unit is actually a special kind of IR-permeable plastic. The WDTV is intended to be fairly sensitive to IR so that you don't need to point your remote directly at it to get a response. However, this means that in the presence of devices that emit IR, you may experience problems with IR interference. A crude solution to this issue is to cut down on the amount of IR that reaches the IR receiver by applying some tape around the receiver to reduce its field of view. This is not an officially supported or endorsed solution, but it may resolve your IR interference issues.
In some cases, the TVs that emit IR do it as part of a power saving feature which can be disabled. If available, this might be a better way to resolve this issue."
Btw, the tape fix seems to be working somewhat. Now I can navigate (pause, FF, rewind, etc.) while watching a movie, but if you press a button immediately after starting a movie, it takes a while still. So the above explanation may have something to it. I hope this is it. If the tape fix didn't work, I would try turning off the C.A.T.S. feature and see.
