Odyssey with self-locking gears. To channel my inner Ted Cassidy, "You rang, madam?" Seriously, I think a lot of folks found out that these generally run better in command mode. And back-drivable gears (as used on the Legacy 6-11334) are your friend!
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!"