If you can get a post card, it could help to determine what is the issue. It sounds similar to the cold boot problem I had with a Socket 7 motherboard. Based on your description, I don't think the problem is with capacitors, most likely it has to do with the chipset.

One thing you could do is to clean well the motherboard from any dust particles, maybe even apply a good wash (but remember to dry it well, blow some compressed air around BGA chips). You could also check the voltages for CPU/RAM/etc to make sure these are within acceptable tolerances. However, the BGA chips (like Intel chip that can be seen from the photo), especially when lead-free soldered would be the weakest part of the motherboard as the balls may eventually lose connection due to stresses and aging. When that happens, your best bet would be (unless you want to torture the motherboard by heating these chips or visit repair shop for reballing/replacement) to replace the whole motherboard, or maybe even the whole PC.

dries_86 wrote on 2020-06-23, 20:20:

Basically there is no video and no beeps. Power LED is on though and CPU fan running and also every second there is an endlessly repeating blinking LED from CD-ROM drive & NIC.

Does sound like no CPU condition.

On the other hand, why replugging it would help I don't know. That sounds more like capacitors. I had a laptop like that, I could only get it to boot the moment the plug hit the outlet, and eventually that stopped working.

But yeah, no idea. =j

Could be a cap issue. I have an Asus P4S800 board that hangs when I try to power it up, and I can't try again unless I disconnect the power and reconnect it. I'm not looking forward to recapping it, because my motivation has been bottomed out for a long, long time.