Looks like torque converter clutch slipping, see if you tap the brake pedal while keeping the gas steady if it jumps up a couple hundred rpm and the surge goes away. Usually I end up replacing the converter for this, and as a failsafe, I usually replace the TCC PWM and Pressure control (EPC) solenoids in the side cover (valvebody). The converter clutch failed in my GTP and the clutch came apart, and clogged the filter so I would loose all gears after a minute or 2 of running. Not sure about that plastic piece is from, hard to identify from the pic, could be small piece of the pressure switch or something. there are not too many plastic parts in our transmissions aside from the filter, the pressure switch on the vb a couple of lube shields and some thrust washers.. so not too sure on that one. Thats just my 2cents.
