Intermittent problems come from poor grounding or poor connector contact. Turn the furnace power off: Check every ground on the unit, any wire that is attached to a screw into the chassis should have a star washer added to give good metal-to-metal bite. Look at the unit and see where separate sheetmetal pieces may join that need the star washer treatment.
Disconnect the multipin connector (if any) and clean it with tuner cleaner you can buy from Radio Shack. You can buy DeOxit D5 from CAIG labs. Spray the connector generously and insert into it's mate several times to burnish the contacts. There may be a relay on the controller that has dirty contacts; spray the contacts and draw a piece of newspaper through the contact face, this will remove oxidation.
Clean the ignitor sensor by rubbing it with paper; sometimes fine abrasive. The ignitor sensor can be found by looking for a thin rod near the ignitor that has a thin wire going back to the control board box.