Some troubleshooting might help with the diagnosis. The first thing you should check for this particular problem is your vacuum lines.
- Are the vacuum lines on the power steering rack properly connected?
There's supposed to be a small unit on your rack with two (sometimes 3) vacuum lines. One's supposed to go to throttle body and the other to manifold. Your idle speed is actually supposed to increase slightly when you turn the steering wheel, not slow down and stall the engine.
- Is the PS fluid actually moving?
At idle, ensure that fluid is actually flowing. If not, check lines for blockages/restrictions and flush/replace sections as necessary. If it's not the lines, then it's either the pump or the rack.
- Is the pump stiff?
Remove the belt and try to turn the PS pulley by hand. It should turn loosely. If not, replace pump.