on my old monte with that engine there was a black metal pipe that goes up and toward the tb from the water pump area. that developed a leak after a while were it makes the 90. worth a look but sounds like your thermostat is going or the fins of the rad and or condenser needs cleaned. best way is to remove the fans, spray from back with simple green and a hose a couple times.
as far as the acceleration i would imagine vacuum leak assuming the dealer knows what they are doing and checked the tps output. it would also be compounded by hot engine, all the rubber would be over expanding. you can test with carb cleaner run the engine and listen to it while spraying lines and gaskets and anything else
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock