Denver & Rio Grand General Question??

[ADCX Rob]

Try it again with smoke off, and report back.

[KRM]

Today I was running our 2011 Denver & Rio Grand General Set when suddenly the loco slowed to about 1/2 speed or less. No shorts on the track just slowed down. I let it sit and it ran fine for about 45 seconds and slowed again. Then it took off again. Then slowed again. Any ideas what is up with the Loco?? What is causing the issue or what part/parts needs to be replaced? It is well cleaned & lubed with very low hours of run time. Thanks in advance! I also see Lionel shows two different e-units for this 6-18749 loco. How would you tell what one you need?