Not that I know much... but check the idle maybe its turned down. mine was down a little low and everytime I turned the AC on the car would almost shut off. I turned it up to 750 instead of 500 an its fine 4 cyliders might need to be around 1000 RPM... I think. Also I think it could be a bad O2 sensor too.
(People let me know if any of my info is wrong, dont want to steer people in the wrong direction)