Skoda Octavia 1 Engine stalling

I own Skoda octavia 2001 2.0. since last month, engine keeps stalling:
1) while driving (for 10km or sometimes for 1 - 2 km in distance).
2) or keeping the car idle for 20 - 35 minutes.
and when it stalls, It cranks but does not fireup. unless, waiting for 10 - 15 minutes.

special cases:
1) when I am driving (over 60km in speed), I feel like the engine is holding back and loosing power. As soon as I accelerate, it powers back (or lets say looks like gaining power to run again) and runs smoothly. This happenes, in couple of seconds (like 1 - 2 seconds.). If I don't accelerate, engine goes off.
2) sometime I drive for 4 hours without having this issue (this is rare other than having the engine stalling twice everyday).

Things I have done so far:
1) I have changed the battery since it was kinda old.
2) got the car scanned (no issues or fault were found).
3) checked if the car still runs while the battery is unplugged (checking if the problem from the alternator).

This happens since I changed the engine oil ( 20w50 ).