monte z34 overheating in one cylinder? perplexed

i broke the timing belt on my 97 monte carlo (78,000 mi.) 2 months ago. I was going only 10 mph when it cut loose. Dealer wanted $1200 so i went to pep boys instead. They suggested replacing the water pump as well while they had it torn apart. When i picked it up, i started overheating within 3 or 4 miles. Never had this problem before. I can hear a popping sound like coolant boiling but when i open the hood, the engine seems cool. I've replaced thermostat, valve cover gasket, throttle body gasket, intake gasket, and had a gm mechanic tear it up again to check the timing and water pump. He says the timing is right on and the pump looks good. He also said he believes the engine is running hot but only in one specific area, maybe even one cylinder. Is this possible? If i shut it down while in the red,then restart, the gauge will immediately retreat back to normal for 20 seconds or so, then begin rising. Its also ok around town stop and go. Gauge swings up and down then, but on the freeway it overheats right away and stays in the red. How could it cool off that quick if the temp reading is right? I've already put 500 miles on it believing that it wasnt really overheating and it runs exactly the same with the gauge pegged at 260 as it does normally. But theres still that popping noise that i just cant ignore. Oh, and pep boys have sworn to me that it really isnt hot, and say replacing entire instrument cluster is the next move. need help

Might have better luck in the Monte Carlo forum? This is the Cavalier/Sunfire/Sunbird forum...