Flickering SES Light

Last weekend I borrowed a friends 22ft enclosed race car trailer. I was towing it empty and my SES light starts flickering. It only happens when going up hills. I would be running between 900 and 1000 degs EGT and 12 to 18 lbs of boost. The light just flickers but doesn't stay on. This also happened over the weekend pulling one of the 5 x 12 U-Haul trailers full with a bed full of stuff too. I was running the same EGT's and boost numbers. Truck seems to be running fine otherwise, no noticable loss of power. I was runnning on the +75hp mode on my bullydog tuner to help keep the EGT's lower. Even on the big hills in southern MO with a load on the truck, I rarely got any black smoke but she cruised right up the hills without any problems at all. I was told this was a 500hp truck when I bought it a couple of months ago but we'll find out for sure this spring when I can dyno it.

I know this is a long shot but does anyone have any ideas what might be causing it?