Diagonal green lines on a green screen

Question

Hi,

My CRT TV (PAL) goes completely green with green diagonal lines (like a diagonal continuous scribble) as soon as I switch it on. After about 10 minutes I switch it off and on again. At this point the lines go and the picture is clear. It happens every time but is now beoming more difficult to get working as more often than not the screen goes back to green. The funny thing is that once the screen becomes normal it stays like that until it is switched off again. What can I do to resolve this problem? I opened my TV (A Daewoo 28") and found the tube to be made by Philips.