I want to thank all of you for your help. I don't think I'd ever have gotten this fixed (unless it died completely) without your help.

After going through all of your suggestions, it turned out to be the fuel pump. Apparently ML320's have a problem with fuel pumps that vapor lock. In my case it was terribly hard to isolate because it only happened after long periods under load.

I've pushed it to the max with the new pump and it hasn't failed yet.

Alan