Engine light on, on my 2001 Saab 9-3 se convertible

I have been having a similar issue. My engine light comes on and then goes off randomly from time to time. When it first happened in summer 2007 (we bought it in January 2007), our Saab mechanics told me that if there wasn't smoke or visible exhaust it was still okay to drive it, and when we did bring it in there was nothing at all (but we had to pay $100 for the diagnostic anyhow.) I've heard that there is some issue with the gas cap and the check engine light sensor, that if it's not on tight it will show up as check engine. I get the oil changed every 3000 miles (less, actually) and it just passed emissions last month (we have emissions every 2 years), it was between episodes!