Hi, I am not familiar with this model number but it may well be a panasonic in neff clothing. How old is it ? it sounds like an old problem with the heater voltage to the magnetron, but that is just a guess, if you intend to persue this yourself call me i may be able to point you in the right direction, however be aware microwaves can and do kill even when unplugged, use a pro wherever possible.
kindest regards
B.E.S. Microwaves
01424 447799
patrick, March 2009