I would try the ignition timing with a timing light mate, that would be one cause if not set correctly. As Mikey said depending on the sensor unplugged from the thermo housing is another. If you use 4EFE, 4EFTE, 5EFE cams time it up to 4E marks. If you use 5EFHE cams use the 5E mark, this is because the cam specs are different, the opening and closing of the valves are different, only by a few degrees though, hence why I don't think its your problem. Personally if its a rebuilt motor, I would have dialed them in using a DTI gauge and an adjustable cam pulley to get the cams spot on.