I have seen loads of weird control board looking faults on and off both with my own machine and a friends one, all seemingly random, but in the end it came down to motor brushes. When these wear arcing takes place and it causes all sorts of weird stuff to happen with the control board. Black soot on the board is common as has already been mentioned, although if it's really heavy, clean the board with hot, mildly soapy water and a toothbrush, then dry it completely before refitting it. If you replace the capacitor, make sure that you don't tear out the plated through holes when removing the old one. I have already seen intermittent problems with this on other machines I have looked at for folks.