Um, not an expert by any means, but are you sure it's the fuel pump not delivering enough fuel? Also, I believe you need to upgrade to the 0411 ECU. I was going to say check to see if you had any SES codes, but not sure how you wired it up.
What I would do is find yourself a fuel pressure gauge and hook it up to the fuel pressure rail Schrader valve. Check to see if your fuel pressure drops any after your engine starts having problems. I bet it's something else like a sensor or your computer. I "think" the GMT-400 may have fuel delivery issues on the 5.3 but at WOT. Maybe, don't know.
Also, doesn't hurt to replace the fuel filter if you haven't already done so. Easy to replace and inexpensive.