Random steering failure

Hello, my car HYUNDAI Accent 4D 1.5 GLS Diesel 110 H From 07-2007 Km driven 103 000 Km
I have had some issues regarding the steering wheel, as it randomly locks/disables the servo motor while driving. The steering wheel gets heavy as if the el-servo is not functioning or another system has failed. The strange thing is that it accurse randomly and can go over 1 week or so before it happens again.
I have had it repaired and they said that the road accel sensor has to be changed as it is the problem, they changed it and said the car is fine. But still the problem persist and occurs.
When I drive and the steering wheel fails it gets heavy as mentioned, but when I stop and restart the engine the steering returns back instant as if their was no issues with the car. And I have to mention that when steering fails/gets heavy the breaks get heavy too.
What can the issue be, they want allot of money to check the car, over 4-6000 dollars they want just to fix an issue that others say is a small error as wiring, I feel that the repair shop is hustling me as the part they changed for 1000 dollars was not the problem as it turned out.

Sorry for bad English, will explain if you do not understand some parts. Thanks for any answer!