Don't switch to mechanical kit..MAN..
Liana has the nost advance CNG kit ...Alll you have to do is some adjustments..thats it..
You said you've had your cars tappets done...at what guage 25 or 28?
also LIANA with BOUCH electronic CNG kit requires tappets adjustment after every 30,000 kms.,along with plug ,if you frequently wash you engine..RPM will drop.only on CNG...Just to tell you,in my properly tunned car i always had to look for as if its on CNG or petrol...also acceleration is good...another advantage is autoconversion from CNG to petrol which no other car offers..
One more thing ..this could also be due to lamda sensor setting..have that corrected and problem sorted..