wheres abouts is this in my magna? is it sumting to do with the ecu or a seperate thing?

its on the bottom of your throttle body it helps the motor keep constant revs at idle......for example when u turn on your air-con the revs want to drop as the motor has to work harder, the ISC adjusts to keep your revs at normal speed.

Fairly common problem through the TR/TS V6 and 4cyl range.
If you need a new one, go to your local dealer and ask for the Mitsubishi/ACA ISC Motor
It is a new range that Mitsi as put on the market with ACA, and about half the price of the genuine one.