If it plays recordings fine in other players but not in myth your disk probably isn't the problem. Especially for PVR-150 recordings which should work on very old hardware. Your problem is most likely:
1. Myth's internal player doesn't play well with ATI. YMMV in all cases with aTI video
2. Most likely you need to play with your playback profiles. I'd actually recommend you try others like CPU++.
for more detailed help, I need to you see your frontend logs from when you are playing back recordings and having problems. You may need to create a custom playback profile to get the best possible picture (I did on all my frotends after playing for a while). I recommend you not change the settings in the stock profiles and create new ones when you want to play, so you always have the stock ones set to their defaults...