E70 4.8 RPM drops and recovers

[Pupsaiden]

Hello there!

I own a 2010 E70 with 4.8 petrol engine. On top of that I have the LPG system installed which makes the car twice as cheap to use.

It's been great for the last two years but recently it started behaving weird. When slowing down, does not matter how hard I do it) it always drops the RPM quite significantly. Sometimes it even stalls the engine if the recovery does not happen fast enough. It might be a full stop or even a sharp turn where I just need to lose some speed where this happens. What is important to say is that, first, this happens both with LPG and with Petrol, so I don't think it's fuel related. Only the stalling is much more frequent when using LPG as it fails to recover that well.
Second, it only happens when the car is warming up, meaning, it does not behave like that when cold, neither it does behave like that when fully in operating temperature (after about 30mins of drive).

I was thinking, could it be gearbox issues, or just a vacuum leak somewhere? How do I proceed from there if there are no error codes stored.

Engine oil just got changed, sparks and coil plugs where changed to NGK set just this summer.

Video of the issue:

[Pupsaiden]

Bumping this one up.

Got some error codes indicating I'm running lean on both banks. Decided it might be CCV valves that's worn out, changed them both and the codes are gone. Yet, the problem is still there. Changed coils and spark plugs as well, did not help.