Ok the problem came back after 23 hours (and 20 more offs and ons). I am sure it has to do with the pressure that is probably not as strong as 1 month ago. I will see if it works to put a tiny bit more pressure and let you guys know if it worked.
Edit: I tried everything today, I lowered the pressure, I tried higher pressure, case of PS3 open and case of PS3 closed, I tried every possible thing, but it doesnt work anymore. Sometimes if I'm lucky the PS3 works for 30-40 seconds and then the artifacts come again.
My conclusion is that the pressure mod works, but I was just unlucky that my PS3 was "too damaged" for the pressure mod to work longer than 23 hours.
Edit 2: I tried to push on my PS3 case with my knee just to see of something happened, and the artifacts went away. Now I put some stuff on my PS3 (24,5kg to be precise) and it works for now (2,5 hours). Its not ideal having boxes and stuff on my PS3, but it works for now.
Edit 3: My PS3 started to have artifacts again, and I tried to put pressure from both sides, and now it works perfectly again.