Please help,

I recently upgraded my stock alternator of my 2007 honda fit. The wiring of the socket is to the stock's specifications. The indicator lamp, ignition and alternator fr are OK. I even swapped the new alternator with my stock's voltage regulator my stock alt rating is 75A. I upgraded to 125A. The problem is when at idle, full load (aircon, lights and fog foglights, heater) and the automatic transmission is in the D gear, my idle drops too low. Super low. Is there any way I can cure this? Please tell me. Thanks and more power

Does it need a tune-up?

No the car does not need a tune up. Everything is practically new, spark plugs, engine oil, tranny oil etc