I have an '01 Chev pickup with Duramax and Allison Automatic transmission with 145,000 miles on it. In a long pull up a grade, the "Service Engine Soon" light comes on and the engine will go into "limp mode" preventing me from exceeding 1800 rpm. All the gauges read normal before, during, and after, and engine performance is normal (right up until it goes into the limp routine). Once this occurs, I'll pull over, shut the engine off, wait for a minute or so, re-start and it resumes operation as normal (until I hit the next long pull). The first time it occurred, went to my mechanic the day after it had happened and there were no "active" codes, but it had "P0093A" and "U1000" stored. I was unable to re-create the problem with a hard run up a 7 mile long grade, but couldn't make it happen. I had approx 5000 miles on a new fuel filter when it occured, mechanic couldn't find anything suspect and said injector operation appeared perfect via his scan tool, so we changed the fuel filter as an easy precaution and decided to continue on and see what happens. Ran it again this past weekend with loaded horse trailer in tow, and same routine occurred 4 times in a 200 mile round trip. Each time, engine would go into limp mode approx midway up the grade. In each case, I wasn't working the engine overly hard, as I could accelerate during the climb at any time. I played around with it a bit on the way back from the trip, and could NOT get it to occur w/o going up a grade. I'd try pulling hard from a dead stop all the way thru the gears to see if I could trigger the event with short bursts of peak torque or peak Hp operation, but it runs like a champ. Only seems to do it 2 or 3 minutes into a long, steady pull. Will get it back into my mechanic this week for another look, but thought I'd check here for ideas. Have noticed no change in performance in past 6 months. Cold starts, hot starts, and fuel economy are great, and no smoke under any conditions. Thoughts?