2000 Pontiac Grand Prix Engine stumbles, tach surges

My 55,000 mi. 2000 3.8 Grand Prix stumbles quite a bit under acceleration then smoothes out and runs okay with an occasional stumble. At highway speeds the tach consistently surges about 100 RPM for a second before falling back. I've noticed it also does this at city speeds with just a flicker of tach movement. The engine speed is easily heard slightly rising and falling when this happens. Around the same time this started I noticed the antifreeze level was falling with no sign of leakage anywhere. At 44,000 miles I had transmission work performed and replaced the channel plate, torque signal valve, pressure control solenoid, and fourth clutch hub so I’m thinking it’s not the transmission. Could this be more transmission trouble or engine related? I’ve heard horror stories about GM’s pink antifreeze causing intake problems; could this be the cause of the surging tach and engine stumble? No codes show when hooked to a computer.

Any help would be greatly appreciated,

Al