Last year I was fighting what I thought was a single cyl. misfire and never found the problem. Now since the outside temperature has warmed up after about 15 minutes of use it doesn't have enough power to move the tractor and will barely stay running. It starts easily and runs fine cold. New injectors, and injection pump was bench tested at shop. They also bench tested the new injectors and said they were good. Compression test is slightly above minimum spec at 330 psi. I've adjusted the valves and removed the muffler to eliminate a plugged exhaust. Injection pump is gravity fed so put an open container of fuel on the hood to eliminate any fuel delivery/ filter issues. When the problem is happening the water temp is 152 degrees at the top of the radiator and 119 at the bottom. Block check fluid shows no combustion gasses in the radiator. Ther is no smoke of any color when it happens. Engine is a DH1101A. I'm an auto tech but I have no diesel experience. Any ideas would be appreciated, thanks.