This is an intermittant problem driving me nuts, which, BTW, is a short trip! I have '99 6.5 turbo, dually 3500, 4x4, auto, 198,000 miles, all synthetic lubes, properly maintained, all stock except new FSD & cooler from Kennedy about 5 months ago. It looses power when ambient temp is REALLY hot. Engine temp is always OK. No smoke, just no power. Example: long trip over the weekend. Truck running perfectly. Outside temp about 89 to 92. Made a pit stop for soda & bath room. Left the truck running for A/C (about 15 minutes). Engine temp at 205 when I returned. got back on the highway & had NO power. Ran like a sick 6.2. Ran that way all the way home (about 2 1/2 hours) When I got in it this morning, it ran fine. It has run fine all day. This has happened all summer. Does the FSD reduce fuel when it (the FSD) get real hot? I can provide more info if anyone wants it. Unfortunately, I only have stock gauges. I hard wired the waste gate once just to check for boost problems. Didn't make any difference. Could the cat converter be swelling closed when it gets real hot? I had that problem with a gas engine once. I'm open to any suggestions unless they involve money...LOL & thanks!