If engine idle rolls up and down, and diagnosis rules out Idle Air Control (IAC) valve issues and vacuum leaks, the Programmable Speedometer/Odometer Module (PSOM) can be at fault electronically. The instrument cluster will need to be replaced, the PSOM is inside the cluster.
Car Problem Reports
Ford E-150 Engine Idle Rolls Up and Down
Ford E-150 Problem
Engine Affected: 4.6L V8