Rpg Maker Developers Group

I want to add random encounters to the world, but not simply enemy encounters after x-amount of steps.
For example; I want to have the characters walk in region 1, and while they're there, there is a chance one of 5 encounters is triggered.

I thought I could do it like this;
When the characters walk through Region 1, a region tracker would play a Common Event.

That common event would then pick a number between 1 and 30. If the picked number is 1, 5 or 7 etc. an event would play. Any other number and nothing would happen.
Unfortunately its not working the way i want to. At first the region tracker event caused the common event to play constantly, since I have it run parallel. Otherwise it wouldn't play at all.
Now, after changing the name, it runs either not at all, or only when I enter the map after leaving a village. And then it only plays once and not again.

What I want is the common event to play when a character stands on a tile that is marked as Region 1.
The common event then runs a number once. If that number corresponds with an encounter, the encounter is played. If not, then nothing happens until the player moves to the next tile marked as Region 1.

I hope it makes sense. Does anyone know if this can be done within MZ itself, or do I need a plugin for this?