Nissan Repair: 94 Nissan 4x4 pickup runs hot only at high speeds., losing coolant, nissan 4x4

Question
QUESTION: 330,000 miles....New.... (timing belt,radiator,water,fan clutch, re-machined heads %26 replaced head gaskets as well as the 3 gaskets around the timing belt) all replaced due to the water pump seizing and damaging the fan and radiator causing it to overheat to the point of damaging everything else.
The temperature gage will reach 34 - 45 hot when reaching and maintaining speeds 65 to 75mph.  This does not occur when I reach and maintain 3500-4000 rpm in 3rd gear.  This makes me wonder if something with the drive train is creating the extra drag on the engine. Could the temperature gauge be out of calibration after the overheating incident?  Maybe a lube job on all drive train parts?  No coolant is ever lost by the way.  

Any thoughts would be greatly appreciated.  Please he me and my beloved truck survive this late midlife crisis. I've invested $5000 over the past 3 years so I'm not ready to give up.

ANSWER: Jeff,

Several thing can cause overheating at speed.

Check and replace the thermostat and radiator cap.

If these are good and you are not losing coolant check the radiator and possibly recore or replace it.

Timing might be set a little too far advanced and if this is the case it will overtemp at higher speeds since the timing is at full advance.

---------- FOLLOW-UP ----------

QUESTION: Forgot to include that the thermostat had been replaced and system is circulating.  Remember that the radiator was replace with a new one. I'll buy a new radiator cap although it appears hold pressure properly.

Why would timing be the issue if the problem does not occur when the engine is run and maintained 3500 to 4000 rpm (3rd gear and only 40 to 45 mph#; but does run hot at the same rpm range #5th gear....65 to 75 mph)?
Could a full lube job reduce friction enough to make a difference on the drag on the engine?  

Thanks again for your guidance.
Jeff

Answer
Jeff,

This would not be specific to lubrication of components and if it is only getting hot at speed the timing might be too far advanced?  Other causes would be a partially plugged converter.  If the temperature sensor was not replaced, do so, to eliminate it as a suspect.  Too cheap not to change it.