Mac Pro won't startup, I suspect the Northbridge chip

I read through several threads and guides here, but I'm a little stuck on getting a Mac Pro 4,1 (single processor) to boot up.

After my machine stopped working one morning, I tried a restart or two where it rebooted and ran for about a minute before crashing. Then it would not boot while hot.

So not yet realizing what it could be, I did a few of the usual things such as reset SMC, PRAM, change the pram battery, clean the memory and clean out all the dust. It was while removing the processor board that I heard the rattle of the spring... and then it bounced out... which I then discovered by reading this iFixit post that it was the Northbridge heatsink stay.

So I bought what I needed and replaced the stays. The Northbridge chip had hardened paste on it, but I carefully cleaned it off and cleaned everything else per the excellent guide and put it all back together.

But I still can't get a startup chime. When I plug the power cord in, I get the quick red interior light. When I power on, I get the quick red light on the processor board, fans blow, DVD drives spin (when they are plugged in) but no chime. I am down to a minimal configuration with no PCI boards in except graphics, and all drives disconnected except the boot drive.

When powered, if I press the DIAG button I get the green PSU PWROK, the amber 5V STBY and a green SYS PG. That's all, the EFI and GPU lights do not come on.

What are my next steps to troubleshoot?