Flutter in idle; serious hesitation

[comanche]

99' Olds Alero 3.4L
(engine recently replaced, sold as 22k)

Idle when started flutters about 500 rpm back and forth 3-4 times.
Severe hesitation on acceleration (not all the time, usually after a couple minutes driving and most often after turning a corner)
New plugs, plug wires, fuel filter, air filter (also tried WAI)
Different EGR valve, MAP sensor, ignition module
Fuel injector cleaner run through multiple times
Vaccum lines checked, electrical connections cleaned.

Hesitation so severe at times it can be unsafe when trying to turn in front of oncoming traffic. Any ideas??