I have problem with my car. My Dealership service dep cannot figure out what is wrong with it. Here is a problem: when engine get worm it stalled and recovering when idling on red. Usually it is starts from dropping RPM from 700 to 400-300. I replaced; air filter, ignition coil on cylinder 1,2 its helped for day or two then problem came back and getting worth. My tech tell me computer has several errors in the log (one of them miss fired first cylinder)
I can obtain complete list of errors if it is needed.
Folks please help! I am stack with this issue.

THis car is 2003 and has only 38K miles. all belts has been replaced, oil changed every 4000 mil. (used Mobile Extended performance oil and filter)
Problems started several month ago after I fueled at gas station owned by Indians. I assume they mix gas with something (water for example)