A number of factors that you need to check :
Do you have a long telephone line from the exchange, if so you may have interleaving switched on. Good for a steady connection to the net, but bad for gaming!
Are you using wireless, and maybe have a dodgy signal?
Are you using, and knowing Abe this is the one thats the issue, a crap ISP like Carphone Warehouse, AOL, or Gyppo.net etc? If so then you should really know better! You gets what you pays for! Get BT Infinity, enough said!