At interstate highway speeds and towing, I get a Check Engine light flash then stay on. "Multiple Cylinder Misfire" code come up when checked. I changed spark plugs, plug wires, dist. cap, and rotor. Fuel pump tests ok. Still comes up. Changed fuel filter, air filter, and used 2 bottles of fuel injector cleaner in 2 different tanks of gas. Still comes up. At regular speeds with no load I don't have any problem. When the tach gets over 3000 RPMs, the problem shows.