XL4 sometimes poor signal on 2 of 4 tuners

I have an XL4 (no mods) that's been running fine for years. As luck would have it, about a month after I opted for a $99 lifetime service on it, I began noticing problems with signal quality. I called TWC and complained, and they connected remotely to reset the CableCard and TA. None of my other (2) XL4 boxes were showing this signal issue. After the reset, the issue didn't resolve, so I asled for a truck roll. After I hung up, I rebooted the XL4, and noticed that the picture was perfectly clear. Great, I thought -- all I needed was to reboot the thing. It must have been having a brain fart. I called TWC and canceled the service call. My joy was short lived though as I cycled through the four tuners. I noticed that with the same channel tuned on all four tuners (as is the case after a reboot), two of the tuners will show a perfectly good picture with great signal in the diagnostic screen, while the other two tuners show no or severely broken up picture with terrible signal strength in the diagnostic screen. I'd hate to think that my XL4 has gone bad. It isn't a cable signal issue since the signal is fine on two of the four tuners. Also, this only seems to affect certain channels as most are fine on all four tuners. Any ideas?