I could really use some technician help. Please take a look at the video below. I've shared this video with the dealer lead tech and he's stumped.

My truck is a 2003 Silverado 2500HD 8.1/Allison. This video was taken a few years ago when towing a 10,000 lb trailer up Snoqualmie Pass in Washington. I encountered this problem only a few times until this last weekend. I now have a 14,000 lb toy hauler and was towing on much longer and steeper grades on Hwy 97 thorugh Washington and Oregon.

The video is a shot of the instrument cluster with audio. When towing on an extended grade the tranny shifts down as usual but after a short while under load at full rpm a ticking noise develops and then the engine gradually loses power until the check engine light comes on. When I back off the gas and let the rpms come way down the ticking noise goes away, the check engine light goes off, and power returns to normal. The video shows this happening above 4,000 rpms. With my heaver trailer on steeper grades this weekend I experienced the exact same situation but at about 3,700 rpm. This was 100% consistent this last weekend. As long as I keep the rpm's way down the truck pulls the heavy load on the steep grade just fine but in the 30-40mph range.The truck has about 70k miles on it. I use it exclusively for towing and winter driving. It is serviced routinely per spec. Any help would be greatly appreciated.

http://www.flickr.com/photos/jmered/5908907485/

Thanks,
-jm