That's just the mystery: no dip, no rise, it is level side-to-side and along the route.
Can you post photos of the track of video of the problem? Its most likely a track problem, especially if the loco ran fine sat for a bit, and now derails. The mechanics of the loco shouldn't change because it sits.
Regarding the sound dropout, I'm 99% sure it is dirty track and/or pickup rollers on the locomotive. These particular models, along with the K4 offered at the same time, 2011, are EXTREMELY sensative to dirty track and rollers. Just because either looks clean does not guarantee that it is.
I usually use Lionels track cleaning eraser first, then wipe the track down with 90% ispropyl on a paper towel as it leaves no lint.
My legacy K4 from 2011 is ALWAYS the first to act up on seemingly clean track. Actually cleaning the track resloves the issue every time. In fact, Lionel made a change electronically in locos released later, so sounds would not drop out so easily.
As far as the smoke unit. These mikados ( as well as the 2011 K4) were produced during the height of Lionels defective smoke fan motor epidemic. Its likely that you may have to replace the motor to correct the issue, especially if its noisy.
( I'm a skeptic about JTs eliminator( at least for this particular model) as it really can't reach the smoke unit bushings unless you REALLY overfill the unit.
The smoke unit fan on this mikado is mounted in a separate housing horizontally and connected to the reservior via clear tubing. It would be a feat for it it reach the motor shaft bushings in any amount to work. Most likely it would just run out of the bottom of the fan impeller housing and ooze out onto the pilot of the loco)
Many of the defective smoke fan motors had shaft bushings that were out of round causing the fan to oscillate around creating excess voltage draw and noise from friction until it eventually quit.
Here is a "textbook example" of a defective fan motor: