2K you gave everything
**EXCEPT**
Your gearing.
Been a long time since I ran buggies but as I recall I'd use about an 18/88-90 for most of them on a stock motor, I'd probably start off with a 14 or 15 for a 13T.
By all other indications - sounds like the brushes are not getting full contact or you're getting some sort of motor interference, are there internal capacitors installed, or did you install them?