Service light and stalling

I took the car (2000 Saturn LS Series) to Auto Zone and had the diagnostic run to find out the source of the problem but it did not help. I took it to the shop and he ran the same test and did not find the problem. He told me that sometimes when repair is done on the car that the computer was not reset and the computer still thinks there is a problem. So he reset the computer and told me to drive it around for a while and see if it solved the problem...No luck. I took it to another shop and he ran the same diagnostic test and found no problem...all codes were fine. He said sometime it could be bad gas. He advised me to add some " TUNE UP IN A BOTTLE" next time I had to gas up and this might help the problem. Well I bought the Tune Up in a bottle and it did still stall on me once. I'm not sure if it worked or not. I haven't driven it much. One other question...How do I check the transmission fluid??? I found the Trans Fluid knob but no dip-stick?