Im working on this model Hp 6550b 6050A2326701 (Diesel 10)motherboard and the 3.3v/5v is switching on and off constantly I have found that Q11 is being switched on and off (Entrip 1 and 2) causing the regulator to output and then stop in a cycle. I have replaced both Q42 and Q11 but still the same . I have also replaced the SIO. Any help appreciated.

Is there any progress with it?

If not, remove Q42 and start mobo without it, then measure voltages in its pads and post results.