Weird cooling issue.
 

I've never seen this before. Started a few months ago. The temp guage would climb alittle and stop. It wouldnt get higher (into the red). Put the heater on and it comes down to normal. Changed coolant/t-stat/cap. Temp doesnt go up anymore BUT.....the part I've never seen before...after shutting down the engine...maybe 45 sec. to a minute, the rad cap would release coolant into the overflow tank. That tells me there's no leaks cuz there's enough pressure to pop the rad cap. It would fill the overflow to where its drippin on to the ground.
Truck starts fine/runs fine. Never been overheated while i've owned it. I'm thinking blockage in the radiator? It's the original radiator w/ 310k miles on it.
Thanks for listening...now back to your regularly scheduled program.

Sorry to say It's very possible that you have a headgasket leak or even a cracked head .I had the same problem- it appeared to be cracked head - exhaust gases went to the cooling system and pressurized it-and there are symptoms you described.I tried everythig you did and didn't beleive that the head was the broblem because there was no water in oil or vice versa.Try and open the rad cap when cold and see if you can smell exhaust gases.

Yeah, I'm thinking the same thing. I've just never seen it happen this way. There's no milk under the oil cap and it runs just as good as when I got it.
Thanks for confirmation on that.

Before I finished reading your post, I was thinking blocked radiator. I had a 77 Corolla that started to overheat. After changing the water pump, I discovered that it was just blockage in the radiator. I had it back-flushed and it helped, but would still get hot if I sat idling for somewhere too long.