Well alright. I've made some changes and now everything is connected via ethernet cable Seems to solve a lot.
I've got 2 PS3s. One is very next to my iMac and the other is downstairs. Both are connected via the same way, only the cable of the one downstairs is longer.
On a certain scene the PS3 downstairs can't handle it while the one upstairs does. It has to be cable length right? So I don't think I can change much more and will just accept the way it is. The scenes are very rare so not a big problem. Though it should be great if it worked nicely.
Now that I think of it, there is a second difference. The PS3 upstairs is connected to a bridge point (Time Capsule) and the iMac as well. The PS3 downstairs is connected to our central router, where the bridge point is connected to as well (Time Capsule). Could that make a difference?