Acceleration, or not so much...

[Waterloo]

Check the jake brake harness when you are in there, it is more than likely time to replace. The heat eventually destroys the harness. It comes in from the driver side center, through the side of the head. You will see it, if brittle or frayed, simply replace. And watch for the plastic clips that hold it in place, DO NOT break and drop one in the head, they can be a real bitch to get out.