Drive train shutters under low throttle at low RPM

When driving under low throttle and when the engine (300C w/5.7) is at low rpm (1000 - 1200 or so) a shuttering in the drive train (feels like transmission but I see rpm move also) occurs to coincide with slight gas peddle actuation. I can also hear what sounds like the transmission shifting back and forth with a bit of a clunk. This happens at several different speeds depending on what gear the transmission is in (30 mph and 55 mph for instance). I work around it by manually shifting to a lower gear which keeps the engine at a higher rpm or alternating between a deeper acceleration force on the accelerator followed by drifting with no acceleration as the car slows back down. The car starts and idles and accelerates perfectly. I thought it might be a failing fuel pump or fuel control system malfunction of some type or even the cylinder deactivation system having trouble but really have no idea. I recently had the transmission serviced (flushed and refilled) at our local dealership. This had no affect.