2 years ago i had a rocker arm break and needed to replace lifters,rods and rockers. since then it overheats only on the highway. I've changed the water pump,thermostat,and flushed the block and radiator. Don't know if it matters i run 87 octane gas. Motor..350 TBI with a truck cam for towing/torque. any ideas???

 

You might want to do a compression test to see if the head gasket is leaking to the coolent system. Also check the timing to be sure it's not running retarded, the vacume advance is working right ( if yours has vacume advance) etc and make sure the engine is not running lean at cruse speeds.

 

You may also want to try hooking a mechanical guage to the motor and driving it. You might be suprised as to what you find. It may be your temperature sending unit failing. another thing....when you replaced the thermostat, did you replace it with a factory stat of 195 of did you put a colder one in? in some vehicles, putting a colder stat in makes the motors run hot at freeway speeds because the thermostat openssooner and the coolant cycles through the radiator too rapidly to efficently cool the motor down.....just a couple thoughts

 

DID YOU CHECK THE FAN CLUTCH...WHOLESALE ABOUT 60 BUCKS