Several reasons including Melgar's. So oil and lube everything. Try spinning the flywheel to completely turn the drivers a complete revolution, feeling for any binding if you go inside.
Sometimes it's tach issues like the gap, or flywheels' stripe condition. Some engines are "tight" and need more break in time. Old grease hardening on stored engines.
BTW: That loose screw was a great catch to help the engine last better.
The space created running them at 10mph for 60' seems like it maybe missing a whole stripe or something similar. Even a bind in a side rod causes issues. I've never had a MTH steamer side rod too tight or binding, but I have had others. I have seen MTH RK starter set linkage bent causing weird running.
I run engines that have slight speed variation and usually put the faster in front. That keeps it from shoving a slower engine off the rails in troubled track areas.
If the speed is too far off it may cause issues. That's why we recommend testing them apart before building a consist.
You could test each bipolar with other known correct engines to see if you have a slow or fast one.
