Rpm dropping problem can also arise due to incorrect tappet/valve clearance setting. RPM shooting to 1500, did u spray anything in throttle body,map sensors are delicate and they dont like desi ustads' most favourite tuning job of "sprays" in TB.
Also the valves need to be adjusted when engine is cold(cold means below 30 oC),inlet gap is different than exhaust. Here almost every mechanic uses same feeler guage(which is already worn out of spec itself) for all the cars out there,and without regard to warm/cold engine. They also dont bother to use cam marking, instead rely on screw driver in spark plug hole, this results in vastly misadjusted valve clearances. My brother's civic(same model ) had this problem of rpm dropping when coming to a stop (no shooting to 1500) until one day I decided to set the valves myself early in the morning when the car had been sitting overnight and engine was Cold . The rpm stopped dropping even on Cng with AC.
