Help, problem only when engine warm.
Truck starts fine when cold. I let it warm up 5 to 10 minutes then drive on flat roads keeping rpms below 1800 till engine is in normal range about another 5 to 10 minutes of driving. Problem starts right before engine reaches normal driving temps. It starts to shudder like its misfiring. Idles fine when stopped, but driving along on a flat road with little throttle, it sputters . Even in 3rd same thing. Give it more throttle and seems to stop, or at least i don't notice it. When truck reaches middle of normal driving range everything is fine again. Not sure what could be the problem. Any input would be helpful thanks in advance.
No mods other than ford ais and cat & muffler delete.