I uped my OpenELEC to the new 4.0 version that XBMC13. I found that it works much better for me. I have an Zoltac Nano AD10 AMD E350 processor. I use to get screen corruption every so often, but that has now gone away. The sound is perfect on all my H.264 .mkv TV shows that I re-encode from the HD PVR that I have. Also, playing any of the movies on it converted from BluRay work without issue too.
Have you tried pulling up the info status on the screen to see in real time what the frame rate, CPU, memory, frame rate and throughput is? I can't tell you what button it is on your remote, but mine is the one beside the '0' on the keypad.
When you upgraded, did you do a clean install as they completely re-configured the back end database and I think I remember them saying that you may run into trouble is you did a standard upgrade.
What device are you running on?
Anthem: AVM60, Fosi DAC-Q5
Axiom: ADA1000, LFR1100, QS8, EP500, M3, M3comp, M5
AudioSource: Amp One/A