[Woocommerce] Add a free gift (product) in the cart when customer add a product in cart -- 2

Hi there,

We have a lot of promotions on our site, and it worked really well but recently we had an issue that appeared suddenly, and it does not work properly.

The promotion we need is : Each time someone put something (anything) in the cart, he gets a specific product for free in the cart as well.

We used a pluging for a long time, and it was working well but now is bugging. Then we made a custom code, but it's also bugging.
So you have three options :

1) Start a new code
2) Fixing the bug of the plugin
3) Fixing the bug of the custom code

At the moment, with the plugin, it shows the gift product in the cart, but if we refresh the page, it disappears. If we refresh the page again, it shows again. If we refresh, it disappears again... and so on..

With the specific code, it was working well. But this code also don't work anymore.
This code will add the specific product and every 3 seconds will add a +1 to the quantity for this free gift product...

So wether you are making a new code, or fixing the plugin or the specific code we made, but we need this promotion to work !
Thanks