You will have to call your provider.
Let me explain. The employee from the cable company sets a resistance (at the pole on the street) of the strenght of the signal you will receive. The employee normally on his visit the first time will put the right intensity of signal for the number of devices going to be plugged. Since you added more devices the signal is not strong enought anymore to give you a good transfer rate for all your computers. Your best bet is to unplug some of the computers or to just call up the company to get it fixed (usually free =) )
I know i will get some responses that it does not work like that, but where i live it does
. Kinda sucks but I gotta live with it