OK thats a good FPS rate anything over 30-40 is well playable,
have you tried asking the other PC users in the house to shutdown their PCs for a few minuites,(or just dissconnect from the wireless network) so you can see if its the PC that you are using causing the connection to lagg.
If when the others are shut down all is better then I would think some P2P program (even when closed) is accessing the net from another PC and hogging the bandwith ,
if there is no difference with only the PC with the problem up and running, then it could point to a number of things ,like interference messing with the wireless conn or some processes accessing the net from the same machine. but would give you a place to start from and help to diagnose the prob quicker
SD.