some of these units have what they call a dashpot on the throttle.
if it is not adjusted correctly it can allow the throttle to drop too low or have a surge in it like what you are describing.
On the higher rpm's i'm not sure what that might be unless it has a mechanical governor on it, if thats been tampered with it might be out of adjustment causing that surge.

Serial number is K005D02373X. It is run by propane(LPG). Thanks.

Serial number please it will help to be able to work out what model fuel system is fitted