I had this happen to be in the beginning, it was the exact same problem as your orange parts show. I fixed it by adding a 120mm 12v fan blowing down onto my electronics board, because I presumed it was caused by the X axis motor skipping steps because of overheating. I no longer have this problem, but my prints are pretty noisy because of that fan. You might also want to try turning down the pot on the driver, as it may be giving too much current which in turn causes escessive heating.
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Charles S.
Software Engineer
Prusa Mendel I2, RAMPS 1.4, Marlin 1.0 R2, Pronterface, Slic3r