The motor mount for the X axis motor seems to be warped and the motor tilted as if the belt tension caused it.  I speculate that the belt teeth meeting the drive pulley teeth at an angle caused the belt teeth to be ground away.

I'm going to shim the motor mount to straighten the motor and replace the belt as a temporary fix, then replace the X and Y axis carriage parts with freshly printed stuff (and eventually, aluminum) that uses linear bearings instead of bushings.  I may even use a small linear guide I recently acquired to replace the X axis guide rails.

I updated firmware using Solidoodle-Marlin_v1-Solidoodle_Marlin_v1_beta   In config.h I specified S3, selected board 81 (printr board rev E), and increased temperature limits for hot-end and bed. 

Result- New temperature limits work fine.  When printing solid infill it prints every other line twice in the same place and skips the adjacent line, leaving gaps in the fill.  The nozzle moves across the part, printing a line, then shifts for the next line, then shifts back on top of the line it just laid down, comes back, shifts to the next next line and repeats.  The brim and perimeters look OK.  Video here: https://vimeo.com/173202575

When printing rectilinear infill at 20% (only value I've tried so far), lines are spaced unevenly resulting in rectangles of varying sizes, not squares:

I tried slicing with Slic3r and Cura and while the tool paths change, the behavior remains the same.  I am using pronterface for host on computer running Ubuntu 14 LTS.