Can anyone tell me how to fix minecraft lag?

Ive been playing the game fine for a couple of months but now after about 5 Min's it slows down and sometimes just freezes. if i exit then start again its fine again only for a few Min's though.This is on single player Ive downloaded it so dont play on browser.im on windows 7 on a toshiba satellite L500-19x, Ive recently upgraded adobe flash player., if that affects it.