Bachmann 67351 N Scale Will Not Go Beyond 70+- Scale MPH

[cbirr]

First post to this forum, hopefully some good information here.

This post is about a Dash 8-40CW bought new from an authorized dealer a couple years ago. Runs fine or so I thought. Yesterday I decided to consist it with another Bachmann Dash 8-40B and started the speed match process using my Accutrack II speedometer. Simple CV2, 5, 6 speed steps. I cannot get this loco to go beyond a scale 70 mph, even at a value of 255 on CV5. Seems to plateau at about a value of 155-160 on CV5. CV2 and CV6 vary just fine with throttle input. CV29 is a value of 34. CV3 and CV4 are 0. I have reset the decoder twice and restarted with no change. My layout runs on an ESU ECoS 50210. I have several other Soundtraxx Econami decoders and they operate with no issues on my layout. Is this an issue with the decoder, or perhaps the motor in the unit? It has never been opened and rarely run, and makes no strange noises. Runs smooth except for the top speed issue. I don't want to spend a fortune on it as it runs fine as it is at part throttle, just can't use it in consist with anything else I own. And of course I'm curious. Thanks for listening.
Ron

[cbirr]

nothing?