We run four brushed Losi SCBs on asphalt. They get hammered from the close combat but aren't otherwise stressed. All the cars use standard NiMh battery packs, either 3000mAh or 5000mAh. One of the cars will now only run at about 75% speed and accelerates slowly with a 5000 pack, but runs normally with 3000. The same 5000's run the other cars without problems. The car's ESC modes have checked out okay. What would cause this? Is it fixable?