2000 4.0 SE overheats

My boss has a 2000 4.0SE, head gaskets have been replaced about two years ago by local shop, before I came to work with him. When I started it had been sitting the two years since work was done due to the truck still overheating. Since then I have changed radiator and two thermostats, with the truck still over heating. the truck will run fine idling, but overheats after driving, sometimes short distances, sometimes all day before it does it. Used block tester and found no combustion in coolant. Auxiliary electric fans working and pulls good through radiator/condenser combo. Any ideas would help. HELP!!!!