Internet laggy when i play games (runescape)?

recently if you play runescape you will know what im saying, i have been trying to kill jad to get a fire cape, but everytime i reach him my internet seems to work fine, but lags for partial seconds here and there, i dont put any other tabs or browsers up and it still does this everytime. it gets really frustrating considering i spend about 2 hours just to get to him, and then i die because of lagg. is there any way to make sure my computer does not lagg? i dont know if it is because of viruses or just my internet connection but im pretty sure my wireless connection is fine, considering it only does partial laggs here and there, not very often. do i need to add something to my computer to make it faster? i have a windows vista computer, plz help. =)