sounds like a convergance issue, not a screen resolution problem.
you probably need a service mode menu.. and if one doesnt exist, then the convergance is hard-wired with electrical pieces on the circuit board that have aged (or other pieces have aged and feeding the circuit less ..or more.. voltage)
use your model number to do an internet search for 'service menu'
if nothing comes up, you will have to take it to an electrical repair shop.