X-axis suddenly skips and stutters

This is an ordbot that has been printing well for 6 months or so on Repetier firmware and host. RAMPS, Arduino and stepper drivers are genuine boards. Recently, the x axis started to stumble and reverse direction like a coil wire had come loose. Here's what I've done. Checked wires-OK Swapped motors and wires and problem still exists. Changed driver twice, no help. Pulled RAMPS 1.4 board and sent blink code to Arduino. OK Reload Arduino/configuration.h. Replace Ramps board with spare clone and problem is the same. Reload Repetier host to an earlier version. Nada Only thing I haven't changed out is the Arduino. Though I have a cloned spare, I can't convince myself that is the problem. Any ideas??

Well, lesson learned, the Arduino was the problem. I hate troubleshooting a problem by throwing parts at it but if you don't know the systems well enough, that's sometimes all you can do. Thankfully none of the parts are particularly expensive.