On my 95 Ram 2500 Cummins 250k miles-

Problem only occurs when at highway speed for long time in hot weather. After a few hours, loses power and misses, have to pull over. Let it cool, all ok until run it another hour and heats up again, same problem.

Mechanic says it's dying P pump. He says new good one is $2500. Ive seen them advertised for 1500-2000. He says he wont use cheaper rebuilds. Does a good P pump really cost that much?

Lift pump was replaced a year ago.

Looking for experienced responses...

Thanks

Jeff G