Thanks Jim for your answer, i'm hosting this forum on a dedicated server (and IPB customer) since 2004 without any issues (even if updates are not easy)
That's the first time i'm running since 1 day error 504... I've modified the php memory limit to 1024MB, max_execution_time to 90sec but that's way too much compare to what it was... (512, 30sec)
Seems like the systems goes in a strange mode., CPU, RAM are totally free