my xbox has started to run slow and jerky in games and dont run as smooth as it used to,

any ideas at first i thought could be the processor over heating so i took the box apart and cleaned the dust out, (warrenty ran out so not paying £80 to send it off, may aswel just buy an arcade) but it wasnt really that dusty inside to begin with so i cant think, unless is the crappy heat sync paste microst put on?