Is it my idea, or the function Update(self) stops working after a while?

It should only stop if the object itself is deleted. What's your script, and what type of particle is it attached to?

It's a script that should freeze an actor, attached to a MOPixel.
When the MOPixel is near enough to an actor, it changes it's position to the actor position, and then, it uses the SetState thingie to make it not move, shoot, reload, change weapons, etc.

The thing is that it works, but before the particle gets deleted, the actor begins moving again.
I've put things to make sure the particle isn't getting deleted before it should, like making it print it's age, things like that.

Maybe it has to do with the Lua AI?

Yeah, I've experienced oddities controlling an actor's actions through use of an outside object. Activities and the actor's script work fine but using, in my case, an attachable did nothing. I don't know if the cause of the problem is the same but it sounds kind of similar, I can only assume, like you did, that it must be the ai.

It seems that the particle sometimes get deleted before the correct time. I guess I'll delete them artificially and check if that fixes something.