Strange Velocity Kraken

[DaveBowman]

Okay experiencing a very strange issue with a large ship, and I've tried everything! As you can see in the video, as I'm burning, suddenly my orbital velocity seems to start decreasing, and Sun Apoapsis just wobbles around a point without moving, no matter what I do. 

https://makeagif.com/i/_bKLhe

I've got a bunch of mods installed including most of USI, Interstellar, and Near Future. Everything is up to date on 1.1.3. Anyone know whats going on here?? Please and thank you!! 

[Benoit Hage]

It's pretty hard to tell what is happening from your GIF as its resolution is quite low. Can you elaborate your problem? Are you already in an escape trajectory? Is your velocity decreasing while burning or after?