agreed with jailbird
cogging -> don't go slow (cogging is very normal for sensorless brushless systems)
tire balance -> take off the rear wheels and slid them onto a free spinning thing, then let the heavy side go down, and add a little weight to the top side till there is no heavy side.

If your tires aren't out of balance, they might have ripped from the bead on the rim (unglued). If you can peel the tire off the rim in any place just use some CA glue (cyanoacrylic (kinda like super glue)) to glue the tires back to the rims. My stock slash tires weren't glued very well to the rim.

as for balancing weight, you can use none drying clay, you can buy specific balancing putty, or get creative....I've used that poster sticky tack before to balance tires. It's worked for 5 dirt runs so far....lol