I think your fuel pump output is marginal. Especially when your truck is in 3rd gear, which is the most load it sees, and consequently needs the most fuel in large supply at that time.
I remember dyno'ing my Mustang when it had a fuel pump that couldn't keep up, and the fuel pressue would take a dive as soon as it was placed under max load.

You should tap into your fuel rail and (temporarily) run a good quality mechanical (Autometer) fuel gauge, making sure you use tubing that can withstand 60-70 poiunds of fuel pressure. Also make sure all of your connections are sealed tight. Then run it in 3rd gear and try to load the motor when monitoring the fuel pressure. I'll bet it takes a dive when under load. Just a thought....

 

I was thinking about it possibly being fuel but why would it take a few laps to happen?

Jerry

 

Well the Fuel Pump is electrical so (this is a HUGE strecth), your pump could be getting JUST enough voltage to maintain line pressure with current from the alternator, but as the engine Heat soaks at WOT, the alternator's output will drop significantly. On a hot day in Florida, I swing a full volt on my 220 AMP unit. I know it's a long shot, but most problems that occur after time tend to be related to heat.

Good luck!