My 1998 Fiesta Zetec only has 33k miles but has started dying while driving. It randomly starts and then doesn't start, so can't be trusted. I recently replaced the fuel pump, fuel filter and air filter. This car had previously only done 500 miles a year for the last 3 years so it's being driven more since I bought it a couple of months ago. The garage that has maintained it for the last 10+ years does not know what is wrong with it. They replaced the relay switch but then it died on them, too. Any clues?