PSON signal to power supply unstable

I am working on an e-machine T2682 that turns on but on turn off, the computer cannot be turned back on again without completely removing power first. On turn off, the power indication light on the front of the machine blinks. The PSON line to the power supply seems to be unstable on power down, either by front switch or by software shutdown. Symptoms are the same with a replaced power supply.