The two sources of picture breakup & pixellation would either be an incoming signal fault, or a hard drive issue.
If the fault is on the incoming broadcast TV signal, then it will generally affect the live feed of all channels carried on the affected frequency - this will not correlate to any block on the EPG.
If the fault is on the hard drive, it will affect all recorded content which is the purpose of the HDD. Paused TV and recordings, both use the hard drive.
If you're certain that only recordings from certain channels are affected, and the live broadcast feed is fine - the only potential conclusion I can see is that certain sectors of the HDD are at fault, and particular recordings happen to have used those clusters. The more severe the problem, the more clusters are affected and thus the less likely this theory is to hold true.