Well, the standard first reply: when anything like this happens I clean the track, but particularly the loco's center pickups and rollers, really well. I find that fixes 90% of the problems like this.
I also have the RK PS2 Y6B. Great loco - my first articulated loco many years ago now, and one I love. Many, many hours of happiness. It's never let me down - when it has clean wheels.