lack of power and low idle when cold

I drive a toyota vios (live in thailand) about 3/4 years ago the car was converted to gas so it can now be driven on gas or petrol. Recently the car refuses to idle when first started up and u constently need to restart the engine until its warmed up a bit. Despite a number of visits to the garage this problem has not been rectified. We now have a new problem with the car losing power after being driven a while especially when climbing hills. What is the matter could it be a carburettor problem?