That's probably as a result of the image processing your Plasma does - I have the same problem, and it seems most LCDs and Plasmas do. Old CRTs pretty much just displayed the image as is, whereas new TVs process the image first, creating a lag.
Hopefully your Onkyo AVR has a lip-sync or delay setting that you can set to put a standard delay of "whatever" milliseconds on the audio to match the screen. I did that with my entry level Pioneer and haven't looked back.
Hopefully your Onkyo AVR has a lip-sync or delay setting that you can set to put a standard delay of "whatever" milliseconds on the audio to match the screen. I did that with my entry level Pioneer and haven't looked back.