Fast dropping idle, Aveo 2008 1,4 LPG

Chevy Aveo sedan, 2008, 1,4 petrol/LPG, manual transmission. I started to have a problem after the partial engine rebuild. The problem manifests itself as fast dropping idle during driving, especially when reducing speed, downshifting etc. At first, I thought that problem is more prominent on clutch pressing, but it seems to be related to speed reduction. I have checked brake booster for leak, but it is ok, also vacuum lines, EGR valve and IAC valve (I actually replaced it, but nothing happend. ECU was resetted, but stil no change. The throttle body wase removed and thoroughly cleaned. Diagnostic is ok, only cat codes, that are not related to this problem (p0420).

Anyone has any ideas what to check next?

Thanks in advance.