Redpoint Motor Controller

I seem to have developed a speed drifting issue, I can not seem to get it right. I have changed the bearing oil installed a new belt but, still have a speed drift over time. When I first turn on the TT the speed is fast for the first five minutes or so, then it settles down and is good for an hours or so. Then all of a sudden it is running slow, today after 2.5 hours of constant running it was running fast when I checked it. I am at a lost for what's causing this, I am sure it is the controller but what can go cause this when all else seems to be working?