How to fix really bad internet lag?

While playing Runescape I noticed that the game plays fine for the first 10 minutes then begins to lag very badly. I am in a low population area with graphics on minimum. This happens every time I get on. My computer has an Intel Core 2 Duo 2.8 GHz processor, 2GB DDR2 RAM, 512MB Nvidia Geforce 8800 graphics card, and I have 20 Mbit/s Broadband Internet. My computer is over 5x the requirements to play the game. Any Ideas on how to fix this lag?