RPMs drop while driving?

Dear all,

My name is Shafique from Saudi Arabia.
Before 3 months while was driving the car suddenly drops the RPMs, and goes to 100 to zero speed, I shut off the engine and start again then it run normally, after 10km again same problem happen. I do the same it start normally again and then after 5km again same thing happens. I showed to mechanic and he changed the throttle, now after 3 months again same problem happens with me. He checked the throttle and he said it is okay. He changed the plugs, but the issue is not resolved. Please guide me to solve this problem.

Thank you