Robyn's advice is spot on. Your RPM is unstable because the solenoid control is unstable. Try the PMD first, and if that doesn't fix it, get the pump overhauled. Have them use your new PMD.
One other very remote possibility: I've seen a pump (one) where the resistor on the PMD was a huge mismatch. This made the throttle twitchy, but it didn't cause a runaway. Your DTC would seem to rule this out, however.
The Constitution needs to be re-read, not re-written!
If you can't handle Dr. Seuss, how will you handle real life?
Current oil burners: MB GLK250 BlueTEC, John Deere X758
New ride: MB GLS450 - most stately
Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510