Antec TP3-550 PSU shuts off under load

[bogdan2014]

I have an Antec True Power Trio TP3-550 ATX PSU for repair and it shuts off under load (as low as 200mA), but only for the +5V, +3.3V and -12V rails. The +12V rail can be loaded to the max with no issues. I'm guessing something must be triggering the protection circuitry, but since there's no schematic available I'm asking maybe someone has encountered this issue before.
I eliminated the usual culprits - bad capacitors. Since the 12V rail can be loaded, I'm also eliminating any issue on the primary side.
I'm digging through the PSU to find out what controller they use, it's incredibly crowded with components.