1991 Honda Civic dies after driving on freeway

My girlfriend's '91 Civic has 150k+ miles on it and just started acting
up. The car runs fine while driving around town, but once we get it up
to high speeds it will just all of a sudden die the next time we slow
down. Most of the time it starts right back up within a minute or so,
but the last time it took about 20 minutes to start up again.

We took it to our regular mechanic, who took it out for a test drive
and never experienced a problem. He said it might be the fuel pump, but
that he wasn't sure.

After reading through newsgroups on the subject, it seems that maybe
the problem is the main relay switch. Any suggestions?