OK, I finally got a dual cell mod to work... YAY ME!!!

The Problem is... I am using a 3.5 from Extreme Micros & the tires spin like bats out of hell when I am holding it, but when I set it down it stutters before going, sometimes I have to give it a push. & it hardly even goes in reverse, if at all. Once it gets going it goes though. It is like the gears are catching or something. but I can't figure out why? Tried three motors & all do the same...

Can anyone help me??