Check the start switches. They will gunk up over time. To clean them use a drop of rubbing alcohol on each side of a white business card (or index card), slide the card between the contacts and move it back and forth until it is clean.
Do not use sandpaper. Even the 1000 grit wet dry sandpaper. The contacts have a very thin gold coating and sandpaper will take it off. At that point, the switches will work for a while, but eventually they will pit from the arcing and then they will become unreliable and will need to be replaced.
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