Steve,
Perhaps much noise was the output (let it be process variable PV). Anyway noise input must be filtered before use it in a DIFF component. A good filter:

PVf =0.5*PV +0.25*PV(-1) +0.175*PV(- 2)+0.075*PV(- 3)

If you use DIFF component you can achieve, sometimes, a much faster and stable PID actuator

Nic