Unless you've moved the unit lately, or the kids have treated it roughly, it's probably not the mirror, which is fixed in place with screws on this model.
You can also take a look at this website here
, which describes some solutions to bowing problems, specifically replacing the convergence ICs. However, this is expensive, provided you can still find parts, and probably isn't your problem, anyway.
Since whacking the unit fixes and un-fixes the problem, it's more than likely a connector that's loose. If you know your way around the inside of a TV, you can unplug, clean, and re-plug the various boards and connectors, especially the convergence and deflector boards. That will likely solve your problem
CAUTION: If you don't know your way around the inside of a TV, DON'T OPEN IT! Even after unplugging the TV, some of the wires inside will have enough voltage to cause serious injury. Better to be safe than sorry.