Grundig CUC5511 goes into standby after few mins

[nihool]

Hi, I have a Grundig CUC5511 and when switched on via remote the TV works fine for about 3 minutes, then it shows a thin white or blue horizontal line for about 1 sec and goes into standby. I then have to wait for about 5 mins before I can switch it on again, but most of the time it shuts off again.... I already replaced the IC676 (LM317) but the problem remains.

Any help would be greatly appreciated!