Ssangyong Rexton running rough

2022 Rexton ELX 800km on the clock running rough, lack of power, between 1 - 2 thousand rpm. Picked the car up about a week ago and drove home, nearly 600km from the dealership. No warning lights, checked by RACQ mechanic, no faults in computer. The problem is in the cruising rev range and when going up hill with cruise on the power fade is obvious until it kicks back and the revs get to around 2k, then it goes like a rocket. There is a noticeable lumpiness in the engine at constant revs around 1500rpm, a bit like a misfire. It doesn't seem to be there all the time. The oil was down to the low notch, wandering if that could affect it, mechanic thought top it up see what happens. I put 1L in and the problem doesn't seem to be there now, tomorrow is another day though.