4.8l suddenly issues.

[H20cooled]

So I have had my 4.8l swap going now for around a year with no issues and suddenly I'm having problems. It will start up just fine and run for 20 to 30 mins then it dies and does not want to start. My setup is:

2001 4.8l 140k miles on it.
returnless rail
Wix regulator/filter
rear install FI tank with walbro pump

The last time it did this I pushed the schrader valve on the fuel rail to see if I was getting fuel and it shot a little out and I went to try to start it and it fired up. So I figure my fuel injectors were bad, I replaced them and added a pressure gauge to the rail and it fired up fine. Drove for 20 mins and it died again. I had my wife turn the key on but not start it and I noticed that the fuel pressure went right up to about 55 and then very quickly went down to 0. From what I understand it should not do this, so I'm wondering what is failing. Fuel pump? Regulator? Something else?

Any help would be great.

Thanks,

Rich