SOLVED -- It's back..... again.

Sorry to hear that Brian - As I said the Mustang and Lightning forums have identified at least 7 confirmed causes and sometimes more than one will be present. Unfortunately what works for one guy may or may not work for someone else.. There are a few causes that seem to be more common though so if it were me I'd go back to the beginning and double check, or even redo some of your old work. You never know, one small bit of debris left on that throttle body gasket that you didn't see before and boom, there's the thing that causes all your grief. I went through all the items on my list at LEAST twice each. And if it weren't for Keno recounting another guy's positive efforts - one of the few posts that I evidently didn't see - I might still have been chasing my tail today   
However, in your case it seems to me that if increasing the fuel pressure directly increases the surge frequency and reducing it has the opposite effect, I might start looking at a possible issue with one or the other of your fuel pumps.