2005 3 slow learning ECU
 

I have a 2005 Mazda 3, the base model with 145,000+ miles. I disconnected the battery to reset the Check Engine light that my aging gas cap turns on every few months when I forget to tighten it well, and after 30-45 seconds reconnected it.

After this, the car when idling would bounce between 1100 RPM and 300. Sometimes it would stall, sometimes it would level out to normal. I decided, after some advice from a real mechanic, to just drive it, on the chance I had reset the ECU and it needed to relearn how to idle.

Well it turns out my car probably road the short bus to school. It was over 200 miles before it it stopped this and started to behave. now it has no problem.

Thought I would share in case someone else has a similar issue, you might need to just keep driving.