Bachmann HO White Christmas Express

[Snuffey]

When circling the track the locomotive loses power and there is some sparking when it crosses the straight section of track. It does fine on the curved sections of track.
This set is only used at Christmas and this is the third season of use.
Has anyone had this occur?

[SteamGene]

Have you cleaned the track and the wheels of both locomotive and cars? Is something dragging that could cause a short as you transition from curve to straight?
Gene