Is the steering servo actually moving back and forth, causing the wheels to move?

If so, its possible you are getting interferance at higher speeds. Ive not seent his before, but its the same concept as the servos twitching when you touch the glow starter to the engine, or when you touch your radio antennae to the engine.

Anyone else have any ideas?

If thats what it is, not really sure how to fix it. Are the wires running too close to the engine? Or are any wires bare (touching metal?)

Dunno..