Radiator? Thermostat? Something else?

I just recently helped my brother replace his head gaskets (1991 V6 3.0 4Runner automatic 170,000 miles). We did extensive service work to the injectors, throttle body, hoses, bearings, pullies, knock sensor, gaskets,etc. ,etc., etc. The truck has not run that good in about 8 years! We were very impressed with our work. Anyway, prior to this, this vehicle has had a slight over heating problem. The truck runs great, like I said, but it still heats up. Upon test driving, we noticed that the temperature indicator was climbing toward the red zone. After 8-10 miles of driving, the temperature gauge indicated "very hot", we pulled over and popped the hood. The engine didn't seem to be excessively hot. I thought maybe the temperature sensor was bad, then I touched the radiator hoses and the radiator cap............VERY HOT!!!
The radiator(original) has been flushed and serviced in the past a few times. The thermostat is relatively new(Kragen Auto Parts) and passed the opening test with hot water prior to installing. What are the chances that the radiator has lost it's cooling abilities? How do you test a radiator?
Any ideas would help.
Thanks.

Try the thermostat again and see if it opens where it should and opens all the way. If so, the radiator is the next thing to clean out. The suction hose has been known to collapse, causing coolant starvation.

it sounds like there is a blockage after the stat,is the stat installed in the right direction?are you mixing coolant and water at 50/50?and at what temp did the stat open during the test.maybee it's opening at too high a temp.i would get a new stat just for the hell of it.

I just replaced the radiator with a new one.
Tested my Stant thermostat with a Toyota stat, and the Stant opened first and the Toyota one didn't open up all the way and took longer to open than the Stant. I took back the Toyota one and kept the Stant.
Engine runs excellent, temperature is good uphill, downhill, AC on, loaded with girls cruising the boulevard.
Original radiator had 170,000 mi. with a few service jobs. It was just time for a new one!

Thanks.

Did you look into cleaning out the old radiator? That used to be useful (back in the old days)...

Yeah I thought about it. But considering, I took the engine apart down to the head gaskets and went through it and serviced many things as described above, I felt it was just like insurance, $180.......might as well.