2006 volvo vnl with d12 435hp keeps derating

hey everyone, i got a problem that my shop mechanics cant figure out and nether can the volvo dealer. its a 2006 vnl 660 with 435d12 and 10spd meritor. the issue is that the CEL comes on and it derates for no reason( apparent to me while driving). i know this gen volvo motor is the most problematic of the bunch. the EGRs have been replaced mulitple times, ECU, harness, and most recently the EGR cooler. the first couple of EGR changes, the truck would run perfectly fine for a day or two and the CEL would come on . The truck was in the shop 3 weeks ago for the same problems and this time the EGR cooler was changed. and again it ran just like it should for 3 weeks now and about 5000 miles. The truck does not use any fluids, all temps are right where they should be, oil pressure is right where it should be. My company has four of these trucks all with the same spec just varing mileage, 2 are fine and 2 are having the excact same problems as eachother and they started having these issue almost at the same time. So... my question is what the hell is everyone missing? there has been ALOT of money dumped in to these 2 trucks with no cure. Any help or possible direction to look would be a major help. thanks alot- bill