Sonata 2002 RPM drops
 

The rpm maybe after 20 mins will drop to below 1..and goes up again. This happens and cause like shutting down. Already changed ECU, one O2 sensor clean the engine manifold , Timing chain changed etc. Twice overhaul engine. Now 170000Km. Once sent to an car electrical repair shop one month there and only found out the timing sensor out. Anyone can advice. I send to a korean car shop..not much use on this matter !

Sounds like you don't do much of the work yourself
Just send it out to have it fixed

It is hard to see through cyber space and find what is wrong with the car
But, lets throw out some guesses...

A failing fuel pump can cause this rpm issue
A faulty Idle Air Control valve can also cause this issue
Bad ignition coils
Frayed wires
Faulty crank sensor
And the list goes on

You don't what to throw parts at it
That can cost quite a bit and still not fix it
Tests