If it happens between 30 mins - 1 hour of your comp being on, open your task manager, click on the processes tab. Then click show processes from all users. Then look to see if your computer is running anything you don't need. (For Example, My computer randomly starts up the windows Media Player sharing service which decides to use like 50% of my cpu, this usually causes my comp to spazz out. when i close it, it works fine.) if you fail to find anything that's making your computer run slow, find the process called Java Dot Exe, (the one that's using cpu) right click on it and change the priority to high. This might help, but you never know. it works for me

also, try using the runescape client if you dont already o:
Best of luck to you sir

iFallen