26PF5521D/10 locks up when changing digital channel (it's OK on analogue but there's no longer a signal).

Sometimes a random sequence of keys on the remote will force a reboot but most times it require the use of the 'Master Reset' i.e. turning the set off at its main switch.

It's running software version 'IdtvZapper_HW260.256_SW2.0.38', over the air 'software upgrade' is enabled & I can't seem to turn this feature off to see whether this is the cause of the problem.

Any suggestions?