Shadow, are you brushless and running a completely tightened slipper?

I had to rebuild the diffs on both maxx's after they started making a "popping" noise...and I couldn't acheive top speed (slippage). I believe this was a result of brushless power, stock slipper tightened all the way down (i.e., no slippage), and a short track w/ lots of on/off the gas throttle action.

I replaced a couple gears, shimmed the diffs, and am looking to order either the Strobe or the RRP slippers.

Not 100% certain that's your problem, but it sounds similar.