I purchased a 2004 chev 4500 in nov with the duramax 6.6.LLY. Truck has been towed three times. Problem is while towing app. 18,000 the engine will suddenly decelerate and max rpm is 1900. If I can get off the highway, shut it down and restart sometimes problem goes away, however problem re-occurs within 5 to 15 minutes.It is now at a dealership and no one seems to know what the problem is. Also the transmission let go with 28,000 while in Baton Rouge and Allison service found the c1 and c2 clutches were burned up. This gets better. After being out of service for a WEEK, I drove off and after app 35 miles had occasion to activate the exhaust brake, which gave off a loud whistle sound and slammed truck into a lower gear. Allison claims they didn't do anything to cause that. Any ideas whats going on??????????

Interesting, everyone says the LLY doesn't have the exhaust brake option. Guess they were wrong.

Use a code reader. Your engine went into "limp home" mode, and can be caused by something as simple as a fuel filter, or something more complex.