I'm not sure if this is the same problem you describe, but I fitted some aftermarket grips which came with new cams. At first I couldn't get the throttle to return, it was like the cables were binding. It was worse when at one lock than the other. Turns out I had to adjust the throttle cable free play - after that I've had no problems.
Apologies if you've already tried, but as a novice this wasn't obvious to me at the time!
Apologies if you've already tried, but as a novice this wasn't obvious to me at the time!