I will be taking my 04' to the dealer on Tuesday, but would like some insight/ideas as to my problem.

04' 3500 with 39000 miles. completely stock. I have noticed shifting problems the last month. i.e. will not shift to next gear when accelerting hard. Problem seemed to get worse as time went on. Tranny fluid level is good. Went on a 2000 mile trip pulling a 6000 lbs trailer. No problems other than the above mentioned shifting problem. Still happened while pulling the trailer. On the return leg of the the trip and within 200 miles of home and @ 1:30 in the morning, on a two lane highway in the middle of nowhere, I was cruising up a moderate grade, at approx 65 mph and the check engine light pops on, and the truck begins to downshift itself to 25 mph and no faster. I pull over put it in park and was able to get going again to 55 and it did it again. I pulled over again, popped the hood and shut down the truck. Started it again, took off and babied it all the way home, with the engine light on. Did not exceed 65 mph but still had to back off the accelerator to get it to shift up.

What do you think?

My ideas....... A heat issue. I was pushing it hard, but i was not pulling a heavy load. Ambient temp was in the low 40's. All the gauges were running normal.

I wish you guys would start changing your fuel filters every 10K.

The fuel filter was changed within 10 K. I do it every other oil change. I also change the exterior tranny filter at the same time.

You could be getting bad loads of fuel. You may want to look into it.

I would first pull the codes, and see what they tell you.

The problem was the fuel filter. Although it was changed recently, it still clogged up. It cost $163.00 to find that out and have the filter changed. You would think they could have an idiot light for such a simple fix. I will now carry a spare filter and wrench in my truck. Thanks for the help...