Toshiba 42X3030D

[ray-murphy]

Hi, I'm in need of some help with my Toshiba 42X3030D.
It has worked perfectly for the past 7 years without a hiccup.
Recently, it's been turning on fine and then after about 45 minutes, or so, it goes to a black screen...with audio still on.
I've replaced the main power board (new) and the master and slave inverter boards (second hand from a working tv), but still have the same problem.
Can anybody please help with pointers as to where I go from here?
Many Thanks,
Ray...