Hi i would like to share my experience with this laptop dell Studio 1558.
After an year of use my laptop started to have this kind of problems like slowdowns and was getting really hot while playing games or simply playing full hd (Mkv, etc) videos thru HDMI.
The heat sink is serving almost all components: motherboard, cpu, video card and like u can see yourself the air intake is really small.
Try to clean deeply the fan and the heatsink, then buy a good cooler stand (enough silent) and u will be surprised by the result.

I hope that this helps you!