My 1989 Mercedes 300E has been stalling over the last 3 months. It cut off on

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the freeway twice, but mostly when coming to a stop or pulling into a parking spot. I paid dealer $220 for diagnostic which was not definitive. They said it "probably needed the two fuel pumps and gas filter replaced at $1200. I decided to change gas brand and put in a fuel filter cleansing solution. Now it has only cut off 2 times in the last month and only at stop signs, but I have not driven on the freeway very much. What could be the problem?