I'm sorry to hear that you're having this issue.
If the problem happens with the one specific input, it may be an issue with the tuner. However, without inspecting it, it'd difficult to determine what is causing the issue.
Assuming that there are no speakers or devices directly next to the channel up/down buttons near the bezel, I'd recommend having the unit inspected by a technician to give you a diagnosis.
I have a 52A550. Ever since I switched to clear QAM basic cable to watch HD the TV changes channels on its own. If I'm watching one digital HD channel(clear QAM no cable box) it randomly will switch to the lowest HD channel number provided by my cable company, in my case 705.
I upgraded the firmware hoping to fix this but that didn't help.
I think there is something wrong with the HD cable tuner (just as I type this it did it again!) This is so frustrating. Especially when I'm watching football and a team is about to score it changes channels, seems to happen at least once an hour.