knaffie

Offroad car with turnkey harness. Ran fine for first 2 hrs yesterday then started sputtering under hard load. Thought I was running out of gas so I filled up. Today ran fine for first 30 min. then did same thing. Found the problem goes away after it cools for a while. Fuel pressure 58 psi at rail never budges. Thought ecm getting hot so took hood off. Better but problem still there. Put on new fuel pump no help. Speed density tune no maf. No codes. Fuel? Electrical? Help please.