M44 Z3 or 318is suffocate momentarily during drive

[didospear]

Hi all

I am driving a 1997/8 M44 (Z3 or 318is). It is 1997 elsewhere but 1998 in South Africa.
It somewhat suffocates during drive. It does not lose power as such because it continue moving at constant speed
Even when pressing gas (accelerator), it does nothing for a while, then suddenly moves.
During the suffocation, the l/100km guage (fuel consumption guage) stays at about 10 or 11 while am pressing gas hard...then suddenly perform again.
I changed fuel filter, fuel pump, throttle body cleaned and airflow meter cleaned. Cylinder head was recently done as well, rather replaced with a used one as my old was bending valves each time after reconditioning. I also changed the lambda sensor as well.
Lastly, the idle is not as smooth. It feels like when it feed more fuel/gas. But the mileage is somewhat reasonable at average on 9 liters per 100km inst city or 7-8 liters per 100km on high way

I suspect maybe air leak but failing to locate it.

Any ideas on what can I check.

Thanks in advance.
Dido Spear