@Ted S posted:
There should be a switch on the bottom of the loco or tender to turn Odyssey speed control "OFF." You could turn it off and see if the loco still pulses and surges. If it does, then your problem is some sort of mechanical bind. If it goes away, the issue is with the speed control circuitry, magnetic speed sensor, etc. Good luck!
"It's not nice to fool Sir Isaac Newton!"
Possibly a cracked/ broken magnetic flywheel ring?
Location of the switch is on page 7 of the owners manual: https://www.lionelsupport.com/...ents/71-1103-250.pdf