It is a mechanical problem, that's why I moved the post!
POSIBLE WHAT YOU HAVE IS RELATED TO THE HYDRAULIC PUMP OF THE ASSISTED ADDRESS. This pump is constantly moved by the engine, but when you turn the steering wheel, it is when it opposes progress, so you notice that the engine (especially in small gasoline) lows back to idle, making maneuver.
Try not to stop and keep the steering wheel strength when you maneuver because forces the bomb, many people do not know.
The assisted address has a By Pass valve that opens when the pump reaches its maximum pressure, and this happens when for example you have a road wheel against a curb and you insist on turning the steering wheel to that side, or when I have said before, you reach the steering wheel stop and instead of releasing 1/4 back, you continue to make strength, it also shows right away.
If it vibrates more than normal it can be because the by pass valve is too high of pressure, because the belt is lazy or the tensioners in poor condition, (tablet is perceived to idle) or because the slowdown is too low and the engine at little force you make, teats to penetrate.
Greetings, anyway, tell me and the engine that carries the Ibiza and possibly you will go right to breakdown.