I have ext GFI plug control by an int switch on which an ext cord is plug to it & intermittent christmas tree lights that trips all the time! What is the problem ?
The plug is brand new & wired correctly,waterproof & so on
That would be my guess, also. Try removing the lights (keep the extension cord plugged in) and see if it continues to trip. If not, try different lights.
If it trips with only the extension cord, remove it and see if it continues to trip. If not, your issue is likely associated with the cord (still could be moisture at the plug). If it continues to trip without the cord, perhaps there is a faulty switch or wiring.