OK ,inow that you have checked all of the obvious things and the display is not indicating the change, then the problem is limited to a drift in the USB section. Your VFO appears to be OK.
Any suggestions as to which component(s)? Perhaps a cap? I don't have a maintenance harness so I'm guessing I'll have to debug statically unless it's on top.
I don't know the specifics of that radio, but my conclusion is from the diagnostics that you presented here.
The USB section is where you need to look since everything else is OK, but only some good old fashioned troubleshooting will get it fixed. It could be something as mundane as a bad solder joint on a critical component for all I know...
Static analysis may not yield much, since it is only off frequency, not totally failed.
Is the USB carrier fully suppressed or do you get some carrier when keying up?
Perhaps someone on the forum who knows the radio may have some further insight for you.