Counterweights when using the PC for polishing absolutely do not matter... a 5 or 6" counterweight will work just fine and as far as I'm aware those are the only 2 sizes they come equipped with.
Remember - its a counterWEIGHT not counterSIZE... so the jump from a 5 to a 6 or 6.5" pad does not require a different weight. You're offsetting the weight, and another 1-1.5" of foam doesn't make a significant enough difference in weight to justify a change.
To the OP... what speed are you running the machine at? Anything at setting 4 or below and it takes almost no pressure at all to stop the rotation of the backing plate. We recommend that all polishing be done at setting 5 or 6.
Even at speed 5 you may see the pad stop from time to time due to the contours of the vehicle... remember the PC is not driven to rotate, its simply the centrifugal force of the oscillation that causes it to spin. So when you come up to a contour or curve that is in opposition to that rotation it may stop or slow, just work on keeping your pressure constant.
The only other thing that could be attributed to stopped rotation is lack of speed caused by using an improper gauge extension cord. When this happens you may notice the machine stop or sputter, as well as get excessively hot after short amount of use.
Check out this video specifically when you have a moment... I explain the rotation and pressure using our system. As I get up near the cowl in some of the spots you can see the pad stop rotating for a little bit... its not the end of the world, just practice on keeping that pressure constant.