Variable plasma picture quality

I've got my Panasonic plasma hooked up to digital cable. A couple months ago we had a lightening storm and the next day I noticed a grainy quality and lack of sharpness to the HD picture I hadn't noticed before. It seemed to correct itself after a couple days.

Then yesterday I noticed the same thing again, but this time no storm beforehand. My assumption is the cable signal is compromised in some way rather than the TV (at least I hope that's the case), but wondering if anyone else has seen this, what may have caused it, and what you've done about it if anything. I don't have another hi def source to use for comparison.

By the way, I don't have a power conditioner/surge protector hooked up yet but will soon. Thanks for any thoughts/insights.