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Sprinter overheats and rpm slows when going uphill

I have a 05 Sprinter 6 cyl gas HD that overheats a little at highway speeds over 60mph and then the rpm is limited so the van slows down to the 50 mph range even if I floor the gas peddle, I usually pull over turn off the engine and let it cool down for an hour and resume the trip. It only does that when going up the mountains while the van is loaded or on extremely long trips. Ive taken it to two mechanics both could not reproduce the problem. It has 250k

You have a cooling system problem and the shop should not need to be able to duplicate it to diagnose it. It could be
a clogged or internally coated (crapped up) radiator,
a weak water pump, perhaps with eroded impellars,
a partially stuck (will not fully open) thermostat,
radiator hoses with collapsed inner liners,
or even a bad radiator cap (coolant boils over below the engine’s normal temperature unless it’s pressurized.

Everything above can be tested. Including the radiator. It can be “thermally mapped” to find clogged vanes or spots coated and not dissipating heat properly.

Whats restricting the rpm?

Perhaps binding due to the heat.

By the way, are you overloading the van by any chance?

Last week the radiator hose busted and when replaced its running like a dream.
The van is never loaded at full capacity but we do have steep mountain roads. Thanks mountainbike

It does appear that the cause of the overheating was #4 on mountainbike’s list.

If, as I suspect, this was an original radiator hose, I would advise you to replace the other one also. If one of the hoses had a collapsed inner liner, the other one is probably not in good condition either–after 250k miles. I believe in replacing radiator hoses proactively–at ~90k miles–just to avoid scenarios like this.

You’re welcome. Thanks for posting back. Good news is always appreciated.

Happy motoring.