New to the forums? I've seen you around here forever!
I can't believe nobody told you to try a smaller pinion. If you can't try a smaller pinion could you possibly try smaller tires? Cogging is simply too much load, lighten the load.
The quark is nice, but I'd check out castles micro stuff. They even have a 9600kv 20mm motor on clearance for $15 and a few 26mm motors for $25.
http://www.castlecreations.com/promo...clearance.html