Originally Posted by Fabrik8

May be a loose coupler in the intake system, or some other type of intake leak. If you have an engine with a MAF, any leak that is between the MAF and the engine can cause glitches. Once you measure the airmass with the MAF, any air that is lost or gained can make glitches because the engine isn't getting what the ECU has already predicted.

There are other possibilities too, it's going to be something mechanical though probably because of the bumpy road stuff. Check that you don't have any cracked spark plugs, and that your plug wires are making proper (tight) contact and don't lose continuity when wiggled, etc.. This is going to be a fun thing to track down, make no mistake...