Try adding a Power capacitor to the battery wires. Mod motors draw alot more amps than any 19t motor, and the power spikes will cause glitching like made.

The capacitor (i'm talking big one like what is on the LRP Sphere TC Spec), helps soak up the power spikes and allows everything to accelerate smoothly with no glitching.

Speed Passion used to do a nice one.

Also, Synthesized receivers, at least for me, are notorious to glitch. Make sure its mounted flat and that all the wiring is neat and tidy.

Other than that, look for loose screws and excess wear on drivetrain. metal on metal can cause interference as well...sometimes you just need a little more RPM to have it cause a problem (IE why it doesn't do it when you run 19t).