Well you should occasionally run on it Petrol too, the 'pure CNG" thing isn't too good I think.
Second great not giving pick.. What can we say, are all the other grears giving good pick? First one will of course not make you feel too bad, but if 3rd and fourth too are being lazy, that means your car's engine is in trouble, otherwise, its the grear box you need to check.
That's of course according to my limited knowledge, others may be able to guide you better.