B5.9 Losing Power as motor heats up

I have a 1992 5.9 in a allegro bay motorhome... When I tried to drive it home from the dealer, it was fine for about the first 15 miles. Slowly it lost what little power it had. It went from cruising at 65mph
to 35mph. The engine temp will never get out of the red cold zone. Everytime you let off the accelerator, the motor would stall. I took it back and they said they fixed it by replacing the fuel filter....
On my drive back home it was better for about 20 miles. Then it began losing power, and by the time I got home, the power was coming in 15 second intervals. The RPM's would jump up to about 2200 for about 15 seconds, then drop down to 1400 for 15 seconds and then back up. I thought maybe I was low on fuel, so I stopped and put in 15 gallons of diesel. This did not help anything. I did notice when I was fueling up, that it leaked between the fuel filler hose hose and the top of the tank.....
Any ideas ?
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wrote:

On a 92 you might have a rotten return hose, that could cause performance problems. I would check all fuel line hoses.
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