it doesn't do it all the time..but when it doe's it seems like it's starving for gas or air only in gear out of gear seems to rev fine..I do have an intake w no cat so I do get a small back fire when it does it but will clear up sometimes I've replaced Cap,Rotor,plugs,wires,distributor and Fuel filter still doing it so I'm pretty stumped.Any Help would be greatly appreciated........