Hi Joe. Although it's true that newer Camelbacks with speed control will run continuously at slower speeds, the way you describe your hesitation doesn't sound normal to me. Even a conventional loco should be able to start and run smoothly at 10-12 scale miles per hour (about 10 seconds to travel 4 actual feet.)
Before you contemplate any electronic upgrades, check the loco thoroughly for mechanical binds. I'm not sure how comfortable you are with taking your trains apart. At a minimum, I would take the boiler shell off and turn the brass flywheel with your fingers for one complete rotation of the driving wheels. Pay careful attention if it "sticks" or at any point becomes hard to turn. Check the side rods to make sure they aren't tangling up with each other, or snagging the hex bolts that hold the rods to the wheels. A poorly designed or maladjusted smoke mechanism could also cause binding. If you don't need that feature, a radical smoke-ectomy will improve performance!
While you're in there, check that the wedge-shaped motor mount is securely fastened to the chassis, and that the motor isn't loose on its mount.
If you still haven't found the cause, you could be looking at a gear mesh issue, or driving wheels that are "out of quarter." These problems require a lot of skill to fix. Adding cruise control electronics may mask the symptoms, but it won't fix these problems, which could be why the previous owner sold the loco. Good luck, and post back if you find the problem.
