lionel sd 80 with tmcc controlling speed

Does it do this in conventional too?

yes

One cause for that is the flywheel sensor has a problem.  Can you run it at any speed?

I repaired one for a customer this year, it had a dammaged flywheel maggnet for the the speed sensor, I repalced the Motor control boad with a ERR board.  Solved the issue

For Odyssey 1, that's probably the preferred solution.  I have some Odyssey 1 locomotives that work pretty well, but I've had others that had lousy speed control and also the Odyssey "lurch" at startup.  I've never understood why such a difference in presumably what is the same control system.

Agreed, I had a similar issue earlier this year and it was a flywheel

locomotive runs in conventional and in tmcc and in legacy but it runs very fast and i was finally able  to slow it down some by punching different buttons on the cab two. it still wants to take off at  throttle up but i controlled part of that with the momentum setting. also hooked 30 cars behind it and that seemed to slow it down some too. maybe if i keep messing with it it will settle down to a reasonable speed, 

Have you tried turning the cruise off?

The magnet ring is busted. You will need a flywheel with a set screw and a magnet, both of which we have in stock in Lionel Service! Thanks, Mike