I have been experiencing troubles with my makerbot rep +.

It is a brand new machine, bus after several prints, printings keep stopping due to a filament jam notification ( error 81).

Actually, the filament is not extruding perfectly, and little plastic bumps make the nozzle go crazy, also causing stringing. Also, at the begging of the print, the filament comes out very curly and then gets even ( see pictures).

I have tried so far

- Adjusting z axis offset

-Resetting to defaults

-Retraction distances

-Different temperatures

-Oiling the filament

-Changing filament diameter

-Cleaning the nozzle

Does anybody have a clue on how to fix this? This is driving me crazy!