OK, here's one for everybody... hey grease, help me out on this one;
I have a 1989 Nissan HardBody that I bought new in '89 and drove it 'till about 6 years ago; I gave it to my dad, then he passed away 16 months ago and I have it now.
My wife started driving it about 2 weeks ago, since she commutes about 100 or so miles daily, and it's cheaper on gas than the r/t or the yukon and it all of a sudden starts running hot.
I've changed the thermostat, radiator cap AND the water pump.
"I" can drive it and it seeems OK, but she told me today that the needle is creeping close to the 'boiling point' on her longer commute.
I have noticed that the clutch is starting to slip; being a "motor-puppet", I can notice this at any given rpm, however, even though my wife has had a corvette with a 4-speed, I'm wondering if she's slipping the clutch and that could be what's causing it to get too hot...
I know "worst-case" could be a blown head-gasket, but my experiences with z-24 Nissan engine has always had a "knock" when the head-gasket needs changing(sounds like a rod knocking); there is no loss of power or water in the oil...

Due to the age of the truck, I suspect blockage in the radiator (corrosion or mineral buildup). If this is the situation, have the radiator rodded or replaced. Refill with a 50/50 mixture of the recommended antifreeze and distilled water.

Have you considered your local dealer about your suspected problem? Looks like a big problem, it has to be thoroughly check. I think you really have to replace your nissan radiator.

I had the same problem with my 1989 hardbody after about 290,000 miles. I suggest you make a very easy low cost repair. Change the tempature sending unit. It is located next to the thermostat housing and is very easy to change.