Overheating: thermostat OK, radiator OK, so WHATíS WRONG???

My car overheats rather quickly. I can drive only a few miles before the gauge points all the way to the red. Twice, I have stopped the car, opened the hood and seen coolant exploding out of the coolant
overfill cap.
My car is a 1995 Pontiac Firebird Trans Am, V8, 5.7 L, 134,000 mi, 6-speed.
I had the thermostat replaced. Then, today, my new mechanic checked the radiator for leaks and blockage and found none. He looked at the water pump, which he said seemed fine.
My mechanic is wondering if there may be a problem with the (his words) "water pump connector to timing chain output." He also wonders if the heater core may be the culprit. As a worst case scenario, he wonders if the cooling jackets may need to be replaced.
ANY INFO IS MUCH APPRECIATED. Iíve searched the internet, but I am not finding a general consensus about why my Trannsy is overheating.
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