No picture on my Dell 790 monitor (17

Question

The monitor picture and Eht died. Only the green led still on the front.
Tube heaters still on and I can get a picture for about 3 minutes from absolutely cold then EHT dies and will not come back with turning set on/off. Picture returns for 3 minutes when switched back on a couple of days later. I have no schematic for this model.
Any help will be appreciated. Tom.