it was : why after a few months where my avg only decreases when i play well and stays at the same value when i play not well or bad, suddenly for a few days my avg decreases / increases /decreases/.....
I'm afraid that we don't know for sure. In the past, top tiers have been subject to newly opened volatility, just to delay progression to a new top tier. But, C is not the new top tier, thus there is no visible reason for such an (unconfirmed) procedure.
I thought it was a rules where avg only decreases after a certain number of ranked rounds played
The rule connected to saturation is a tad different, but you describe the usual outcome. Alas, rules are at the discretion of WGT - they have been changed a lot around here.
My experienced guess to your observation is a simple flaw, initiated when the conditions for TC were implemented.
That happens when QA is lost in "Bielefeld" ;)