The only time I've seen this problem as well was with one of the work trucks, it turned out the armature clearance on one of the injectors was way out of spec not even a .001 filler gauge would fit, shimmed it and the lope went away and all was well.

 

Okay, just for reference, I also have the fuel crossover and a new pump my pressure is at 71. I've seen it dip to 67-68 when towing here in CO. Good luck figuring this out!

 

I have the original pump and an FRx and I get about 62 at idle. I've seen it dip as low as 57 under heavy throttle going up a long steep hill in OD.

 

Agree with getting a scanner. Auto enginuity or torque pro. In my opinion, everyone who has one of these trucks and likes to do their own work should be able to scan for codes and read the essential available information from the engine. It makes it so much easier to both do your own troubleshooting and also ask for help on this forum. Guessing gets really expensive.
Larry