I had some problems with my sunhokey prusa i3 before. While printing PLA, my extruder motor skipped steps at printing speed=60mm, if temperature was <240C. At some point, I got tired of it, and decided to give more current to this stepper (I though it was on 1A). But then I actually measured it, I found current was set on 2A already, and 2A is actually maximum allowed current for this driver (pololu a4988). But while tinkering with driver, I saw this small radiator, glued to a4988 with some black adhesive tape, moved from it’s place due excessive heat, and almost shorted some pins of pololu board. So I took this radiator, removed black goo with isopropyl, and glued radiator back to a4988 with special glue for radiators. After I assembled everything back, I noticed I have no skipped steps even on 200C. So, it’s looks like driver was not working on full power due overheat protection, and proper thermo-glue fixed this. After this, I glued all drivers in the same way, and also attached small cooler to electronic board. Maybe this information can be useful to somebody.
WARNING: pololu board is extremely tight, some SMD capacitor are almost on same level, as a4988 chip, so it’s very easy to make short-circuit with metal radiator. I used kapton tape for insulation SMD parts. Use on your own risk.