Just based on my limited experience.
1. I agree that you may have a power issue. This is easily solved by adding more power taps into the track. (By the way, do you have any really big dual engine diesels? If they can run at high speed on the track for long periods of time, then maybe you do not have a power issue. I would check this out first.)
2. Remember that Steam Locos, unlike diesels, generally have three and sometimes four sets of steel wheels situated really close together. Going around curves, it is easy for them to have some of these wheels lose contact with the track if the track is bent just a little off its specs, or if the wheel spacing is a little off its specs, or if for some reason the track is heating up, or the wheels are rubbing hard on the track and shedding metal dust onto the tracks (black color) as it travels. I had a NIB Hudson steamer that displayed these issues, basically because all of my curves were the tight 031 curves, and I could never really resolve it. Some steamers are just plain engineered to run on 048 or larger curves, even if the specs say they will run on 031. I solved the problem by selling my Hudson to someone who had big open runs and wider curves, and it ran without problems.
(Try taking your steam loco to a layout with big wide open curves, and see if it runs without problem. If it does, then there you have it.)
Just in my limited experience, I have never had a diesel that gave me any running problems (except a tiny BEEP, and the problem was resolved by removing the traction tire.)
On the other hand, I have never had a steam loco that did NOT give me some running problems. All of those wheels, all of those bouncy light trucks in front and rear, all of those tightly packed wheels in the center. You just have to sort through the problems one by one, and if you find that you can't get it to run well on your particular layout, then get rid of it. It is just too frustrating to deal with.)
Just a few of my thoughts to help you analyze this.
Mannyrock
