1999 Hyundai Starex transmission failing!

Best way to fix the problem plss.
i have this Hyundai Starex 1999 model, automatic transmission problem is when I turn the headlights and the aircon ON after a several minutes of driving, the problem occurs. For my experience in driving this car it feels like it always stay on the low gear and it don’t change until I turn the aircon off.
But when I use it during daytime and I only use the aircon, the problem won, t occur. I think it is a circuit overloading, but can you give me more precise answer. PLSS help me!

i am an amateur, or I can say old fashion mechanic that only know the manual transmission. Please help me, other mechanic here can’t solve it also, plss help. Thanks!