The sims 3 suddenly overheats my pc please help(read details before answering)?

I have a dell studio xps laptop and i used to play the sims 3 on it all the time and its settings was always on high but lately after 2 hours of gameplay it heats up and its graphics are taking forever to show up so i end up stop playing turning of the pc and wait to cool down and every thing is on medium or low my sister s pc is worst then mine and it works perfect better then on mine please help