Engine's dieing out at bottom of long hills.

Howdy all. I'm new to the boards and new to trucking and I'm having a problem with my dump truck.
It's a '90 Kenworth with a Cummins Big Cam III or IV? NT850?
I'll do my best to describe what's happening.
After a long run down a hill I'll step on the throttle at the bottom and the engine's very slow to respond or won't respond at all as I pump the pedal.
Usually nothing happens on the first couple of pumps and then when it comes on it revs up and down very slowly for a few seconds. The longer the hill the worse it is. I was running down a mountain road last week and pushed the clutch in at a switch-back and the engine actually died. Whether or not I'm Jake brakin' doesn't seem to matter. I'm getting used to it now and when I'm ready to accelerate I have to pump the pedal a few times before she'll get going again.
No other symptoms other than sometimes I've noticed what seems like huge amounts of positive pressure in the fuel tank. Don't know if this is common or not or if it's related. It's got a non-venting fuel cap that I've thought of venting.
Other than this she runs tip-top.
I'm fairly clueless to say the least but it seems like a fueling issue of some sort. Motor's running at 1500 -2000 rpms for a long time with no throttle input and it's running out of fuel??

thanks in advance all-
bob