car running rough

My car was fine then one day, it started idling rough and lurching. Shortly
thereafter, the check engine light came on. Took it to the garage the next day
and the light was out, but they checked the code and said it was a random
emissions code, and if it persisted or the light came back on they would have
to put on the machine to test. Said it could be a bad plug or coil. In the
meantime I had the front flex pipe repaired as that had broken before all
this.The car seems somewhat better. Not doing it as often. I did put some dry
gas in just in case and fresh gas from a different station. I am on social
security,and have to watch every penny. Any advice? My husband took care
of the cars , but has passed away and I know nothing of cars. I can't afford to
have all kinds of tests and workups done.