Service light comes on in 1995 940 sedan 4-cylinder. goes off after about 10 minutes. Is this a problem? Can it or should it be reset?

The "Service" light on these cars indicates that its regular time for service, as in an oil and filter change. It is completely normal and is not a problem. If you have had your oil changed recently (not at the Volvo dealer), they probably just did not know how or forgot how to reset the light.
In order to reset the light yourself, pull up the rubber plug/cover to the lower left of the speedometer. Underneath it you will see a plunger/peg. Use a pen or screwdriver to push it down. That should turn off the light until the next service is required. If that does not turn off the light, then some minor issue is wrong, likely with the button. It does not indicate any actual mechanical problem with the car.

Service light comes on whenever the computer senses some problem with the engine, it can be a small problem with one of the sensors but it might also mean serious trouble. get the car diagnosed.