Walthers purchased LifeLike in 2005. For a couple of years after that, they sold products under the "Walthers LifeLike" brand and kept the nomenclature. In 2008 (IIRC), they rebranded, so Proto 2000 became Walthers Proto, Proto 1000 became Walthers Mainline, and basic LifeLike became Walthers Trainline.
The Proto and Mainline locos use the same mechanism; the details and features are different. Not so with the Trainline, which uses a cheaper motor and drive train (as well as being still less detailed). You first said that this loco is a Mainline, now in your last post, you said Trainline. Makes all the difference.
I have 6 Mainline locomotives, and they're all silky smooth runners, so I'm betting that what you have is a Trainline. I have two of these. One runs very well, the other is problematic. While a good cleaning and lubrication may help, and you should give the drive train a thorough inspection for burrs or other issues, the only real solution is to upgrade the motor and drive train with higher quality ones.