Engine brake problems...
 

As of late my truck has been having some major problems with the engine brake. I have a 2005 Peterbilt 379 with around 300k miles on it and it has a Cummins ISX (EGR equipped).

When I am going down long and steep grades, the brake will come on but only under a minimal setting regardless of the actual position of the "C" brake setting. After continuing to accelerate down the hill, it will then kick up to medium or high.

This is very prevalent going down grades in Nevada, California, and Oregon (I am a western 11 driver). When it tries to engage the medium or high setting, it will make a lot of noise (can't describe) and it then feels like the engine is unbalanced. After cycling the engine brake, it will then start to actually work correctly.

It is currently in the shop at Cummins and they are having one heck of a time trying to get this figured out. Any ideas?

Thanks in advance.