Shorted capacitors on -12V line? Something that would trip the PSU but not right away? Or just too much power draw cause it to cycle, P4 aren't exactly known for being energy efficient. Could also be failing capacitors in that PSU, just marginal enough for other systems but not for this one.

yeah both power connectiors are in. tried it with my old board and it works fine. it is also a p4 board. tried it also with a more powerful PSU with 420 W. the same. I also plugged in the hdd. you can hear that it wants to start but it cannot. so every second it starts and goes out same like floppy. so must be something wrong with the power supply on the board i guess

Inspect all the motherboard capacitors near CPU socket. While P4 mobos used decent caps, the temperatures of the CPU, and voltage regulator as well, were so extreme that even those degraded after a few years of use. If they failed and there's too much ripple on the power rails now, that could easily cause some parasitic oscillations in the VRM and produce current spikes that would trip PSU into restart. If you care about this board it's probably a good idea to replace all these capacitors - that's at least the repair that can be done.

i do not have a multimeter, but optically everything looks ok. ok i think i have to look for another board

one thing i do not understand, shouldnt atleast the hdd and the floppy work constantly since they are not controlled by the board?