I sounds like you have an intermittent problem which often times is very hard to diagnose.
You are going to have to do some troubleshooting on your own to determine what part of the control system isn't working.
If you are uncomfortable with doing testing yourself, find a good service repair facility or mobile tech.
If you feel capable of following troubleshooting procedures, **HERE**
is a link to the Suburban Furnace Service Guide.
Best of luck and let us know what you find.