Subaru&me

I have a 1990 Legacy wagon, 153,000 miles. For the last three years or so, it stalls after being driven for at least 30 miles when I slow down for a stop light or in a parking lot. It starts right back up again without any problems. Also, the check engine light comes on and stays on for periods ranging from 5 minutes to an hour. This has been happening for about 4 years, about every other time I drive. One mechanic I took it to said he has seen this a lot in the Legacy and that it is just a glitch. Is this true? The car had a major tune up a couple of years ago. thanks for any info you can provide!