04 cummins losing rail pressure

[Foster#17]

I noticed today when I was cruising up a mountain my rail pressure kept dropping off? It usually runs around 18k and at the start of the mountains it's good but after about 10 seconds you can watch it drop down to about 13k?? As soon as you get on it, it comes right up to where it should but when you go back to cruising it's still low. It always returns back to normal going down the other side of the mountains? It's like when it loads the engine hard it's losing pressure. I have everything listed in my signature so fuel is not an issue, anyone have any ideas?