Voltaire,
The computer only changes player for me when,
1. A cross is coming in from the computer, in which case it changes to the defender best placed to clear the ball.
2. I pass the ball. (duh!
)
3. One of my AI teammates steals the ball, in which case it changes my control to him.
If it is changing at any other time than this (I'm pretty sure this is the only time I get the player changed?), then you are definately set to either auto, or semi auto changing. It sounds like semi-auto, because you still can change by pressing R1 (don't you mean L1?).
My instructions before should be ~somewhere~ around the mark.
But
definately try and change it to fully manual switching. The semi-automatic switching was something that I changed almost as soon as I played my first game of ISS Pro Evo, and I can understand it pissing you off.
Hope I've been of some help.