Monitor turns off after 30-40 minutes

Monitor turns off after 30-40 minutes I have a quite old LG LCD monitor, after i am using it for about 30-40 minutes the monitor screen goes off for a second then comes back, after another 20-30 minutes it is going off again and this happens constantly but always needs 10+ minutes to happen. Tried it with multiple computers, it is always the same. I opened the monitor and checked the capacitors but none of them are bulging. What could be the issue then? Thanks in advance.