What can cause this on my phase 1?

What do you mean Hunting on tickover... tickover is cranking I guess? but hunting?

when idle, when the engine is running but the throttle is not pressed. my engine increases and decreases its speed.

Maybe you idle airflow actuator is not quite working as it should. You cab try to clean it a bit.

+1
otherwise it must change ... ~150 € at citroen in france ...

It's in lots of PSA cars like the XM, 306, etc. Take a look at a local scrappy and you'll save some €€€€'s.

is this the idle thing u have put a filter on and connected to the main intake?

That's right dugy.

Hi,
I had the same problem when I first got mine. After a lot of head scratching it turned out to be a small split in one of the AVAC pipes that was very hard to see / find. Would sugest checking them from start to end, if they look old and worn replace them anyway, you can buy a length of the pipe for a few quid. Hope this helps.
Cheers

Yep it was the vac pipe going to the ecu, was split and not secured correctly. fixed it and she now runs great! thanks for your posts!