Inconsistent vertical walls on a Robo 3D R1+
Mmm... you've tried lots of obvious things. I don't have a printer the same as yours, so my help is generic. 

How about a thorough check of extruder and filament path, since it may be just inconsistent filament feed? I've seen something similar happen when the filament reel was not rolling freely. Less likely, but also check that the frame is still solid and nothing has come loose.