Where is the IAC unit located?

What is your question?

A few weeks ago while climbing a steep hill @~50-55 mph under a heavy rainstorm a sudden train of pushes were felt & speed came down to ~35 mph. The symptoms repeated at a second hill right after the first one. It has also happened in flat roads after that. After checking the codes it says: P0507. Have tried at the internet but the code-vehicle combination has elluded me; all I find is another problem in the vehicle or the problem at another vehicle. How to proceed from here would be appreciated. Keep in mind that I'm not a mechanic but have worked as an Electronic Tech., also many years ago repaired my 20R engine from a Celica-1978.