Gen 1 Makerbot Dual X axis issue

I have an original Makerbot Replicator Dual which has been my workhorse for years. Firmware was upgraded to Sailfish, extruders rebuilt, and suggested mods made to the Mightyboard. It's connected via OctoPi as most people do now. It's been flawless for several years, just regular cleaning and maintenance.Lately the printer has been exhibiting strange behavior during prints, i.e. not moving in the +X (to the right) direction. This is also exhibited during homing, as it's not been homing on the X axis. Whether I use the "Home Axes" function on the printer or a G-Code "G28 X Y Z", the Y & Z axes home but the X remains stationary. I can move the carriage along the X-axis IN BOTH DIRECTIONS using the jog functions on the printer or via Octoprint, so X isn't dead. As an example starting from X-40 Y40, a G1 X40 Y40 will not move the extruder, but G1 X-80 Y40 will.Any ideas where to start looking? Stepper motor? Driver module? New printer?