thanks guys for your coherent responses.. i'm proceeding in a few days back to the guys who claimed to be the best in town .. i'll post the final outcome here for everyone's benefit..
Meanwhile, if still someone has any valuable info with regards to the problems i discussed above .. ur reply will be welcomed.. I'd only express my little doubt here though, that once the car is running on petrol, apparently the emulator and advancer should cease their functions as their participation is only needed with CNG as the principle fuel supply. So.. if the engine is back on pet why should they (or anyone of them) still be a contributing factor into the whole combustion process. Maybe that can lead to a key.
Secondly, my next calculated estimate has its focus on the 3000 RPM. I guess the VVTi engine of corolla possibly has some electronic fuel requirement fed into its chip in a way that jets free fuel into the engine after 3000 RPM owing to its higher / faster combustion requirements. Perhaps it could be yet another reason why Toyota asks us to change gears whenever we touch 3000 rpm on the way up (as per owners manual) for possibly anticipated/desired fuel economy. If this is verified by some expert, then it becomes rather obvious that changing of the CNG has now disturbed the petrol supply settings of the vvti fuel injection. As a result, once VVTi varies / turns up the fuel's injection at 3000 rpm, the petrol supply doesn't reciprocate accordingly thereby causing the engine to starve there on and respond in a way i described earlier. And if I take all this on board, my question will be how can that be corrected? The engine check light is also on just for info.
Any well guided comments will surely serve to help a lot I'm sure.