As mentioned, better to sacrifice a bit of high altitude performance for a safe range in the 2000M to sea level area. If the stock Main Jet is a 132, you know that jet is perfect for Sea Level up to about 3 to 4K meters. Above that you would benefit from the 125, as you found out.

But once down below 2000 meters, I would go BACK TO STOCK main jet.

If going UP and Down constantly, try a compromise. I would try a 128 if available. Won't be as good as the 125 up high, but most likely OK at sea level.

But to be SURE ... once down below 2000 meters, pull the plug and have a good look. If it's white or has NO deposits, you may be running too lean. So go up one Jet size. If the plug is a very light Tan (or light Gray) ... then you are PERFECT.

It will be sort of normal for your plug to get more deposits and darker color up high. That's OK as long as it's not JET black and covered in soot.

The other thing you can vary is the air flow. When UP very high, if jetting is a bit Rich, try to OPEN up the air box, remove any restrictors or take side cover off. Run better? It should! Now when you go back down, put restrictors and side cover (if you have one) back in.

Reading plugs is a bit old fashioned these days but it's how I learned and what I know. But with modern fuels it's much more difficult. They burn so clean, hard to tell what is going on. USE CAUTION.

I doubt the fuels in S. America are as clean as US or EU fuels however, so reading your plug should still be a viable option there.

Good luck!