Hi guys, new to the forum. I own Mazda 6, 2006. with 2.0 petrol engine 108kw and a manual transmission

I have the following problem, might be a bit difficult to describe and understand, I'm having uneven acceleration, like in very fast intervals, feels like the engine is pumping. But only in certain cases, like when I cruise (it runs fine) and I put the throttle to like 50% but fast, it will then start to pump. If I floor it, it's pulling normally and really hard, no power loss. The problem is more noticable with more engine load, but it's driveable, if I drive easily or floor it when needed it's not an issue, been driving it for a long time like this without any changes in engine performance and any problems occuring.
There are no error codes in ECU!!!
Anyway, I've replaced spark plugs (multiple times) and currently on NGK LPG4 (because I have lpg installed), all filters and fluids replaced with all the best ones from Eneos. I've replaced VVT solenoid as a friend had a spare one (although there was no error code), cleaned throttle body separately, also later used a can of Liqui Moly intake cleaner to clean the whole intake, cleaned catalytic converter, cleaned MAF sensor, tested vacuum hoses (all seem fine) and variable intake manifold seem to operate normally, no oil leaks anywhere, transmission works as a charm.

As I mentioned LPG, I've first had this problem when I had no LPG installed and I was thinking that the problem might be somewhere in gasoline line, so to exclude this possibility and to have a better fuel economy, got STAG lpg system installed, the car runs totaly the same on gasoline or lpg, with all the same issues and with close to same power levels.

Any ideas what I should look into next? I was thinking that maybe VVT actuator might be bad, but I think it would create bigger problems a long time ago or at least any kind of a noise, but there aren't any strange noises and the car idles fine and cruises fine, and if I accelerate gradually and easlily it also runs normally.

Ah, also to mention, when the car starts pumping, if I leave my foot at the same possition of the throttle the pumping starts getting stronger over time (when it starts it is hardly noticable), but if I floor it, it immediately stops pumping and becomes a beast.