This may or may not apply to your situation, but my DP had a 100% clogged hose from the radiator to the expansion tank causing a similar issue on hill climbs. I only resolved it purely by chance after replacing the thermostats, cap, and pulling the water pump.
1972 Winnebago Brave D20T
1993 American Eagle 38A (Currently dead in Bakersfield, CA. Know a diesel mechanic?)
2003 Chevrolet Tahoe w/Remco Disconnect - SOLD