if I turn the box on and im on a bbc hd channel after 3 -5 seconds the box reboots and does this again and again unless I change to a non bbc hd channel

once the box has been on for 2 mins or so and watching any other channel (even itv hd!!!) I can then select a bbc hd channel and it behaves as normal.

im thinking power supply on the way out, I did change it about 4 yrs ago..

any other possible causes?