A guess - the Vehicle Speed Sensor (VSS) is failing. The engine management
system likely cares how fast you are going, thus the Check Engine light. It
could be worse. My wife had a friend whose late model Buick's VSS failed
entirely while waiting at a light; the CEL came on. When she took off the
electronically controlled transmission wouldn't shift out of first gear -
after all, what gear should it be in if the car is not moving?

In your 91, the codes are probably gone after the light goes out. If it
comes back, retrieve the codes (http://tinyurl.com/y6scej) and record them.
You will need a small shorting jumper - a short piece of small guage wire
works well - to substitute for the "Code Reader" the manual describes. As
mentioned on sheet 18 of the pdf, the connector you need to access is under
the dashboard on the passenger side.

Your approach to the "check engine" light is approximately right. The light
is there to notify the driver of emission-related problems that often don't
show up as operational problems; there are no critical warnings (like engine
temperature or oil pressure) involved. It is really saying, "I have
information for you." Rarely, the failure may cause the mixture to run very
rich and prolonged hard driving could damage the catalytic converter.
Because of that I recommend avoiding the freeway, or at least following
behind heavy trucks if freeways are unavoidable, until you can determine
whether fuel economy is near normal. Once that has been determined
restrictions are off, but the light should be investigated when practical.
If the light is accompanied by lots of misfiring or things like that, of
course you have to respond to the other symptoms.

Mike