Hey,
Sorry for the delay! I was out of the office yesterday, but the guys on here pretty much hit the nail on the head... Just in case anyone else experiences the same issue:

From what you're describing there's 1 of 2 things that could be going on, so it should be a fairly easy fix.
To figure out exactly what's going on I'd need to have you try to run the motor detached from the body and see if you are experiencing these "surges" while the motor is alone. If the motor runs fine on its own, but boggs down in the body, it's normally a lubrication issue…
If the machines haven't been broken in, the grease lubricant that is used can build up and cause additional friction which is what will make it seem like the machine is speeding up and slowing down, but luckily it's an extremely quick fix to get the machines up and running 100% again.
All you'll need to do is add a couple of drops of the oil lubricant to the machine to cut down the friction of the grease as shown in this video: http://www.youtube.com/watch?v=REDrB...ature=youtu.be


This should fix up the issue, but in the rare case that you experience this issue with the motor alone then it could be an issue with the motor itself that we'd need to look at, but 99% of the time I've heard this happen it's been a simple lubrication fix …