MacBook overheating and fan spinning too fast

Hello! I've had this problem on my MacBook from 2008, but its never been this bad. On SMCFanControl, the temperature is steady at about 70 degrees, and the fan is running at > 6000 RPM. Every now and then (especially if I'm watching a YouTube video, or Netflix) the computer will automatically shutdown without prompting me to do so. My guess is that it was overheating and shut off in panic mode.

I've checked Activity Monitor, and it doesn't seem to show anything taking up a lot of CPU power. Also, I generally only use this computer for internet, word processing, and TV (through Netflix).

Any help you can give me would be appreciated! I'd prefer to work this out without having to go to the Genius Bar. Every time I went to them about it, they just did a system check and showed everything was running smoothly. I've replaced the battery several times, thinking to work with a simpler problem before assuming it was overheating.

Watching videos on a MB, especially flash, the fan is going to be spinning. There's no getting around that.

Having said that, with a 5 yr old MB, would highly suggest opening it up and using a can of compressed air to dust out the insides (the fan in particular) and see if that helps. Just had to replace the fan in my wife's '07 MB - however, her's was just beginning to make screeching noises. But that's not going to keep the fan from spinning at max speed while using flash - she plays some flash based game - so her fan always kicks in fairly quickly after starting that game.

Head to iFixit for step by step, with pics, for opening it up.
Think they have a fan replacement guide for just about every Mac made - use that one - though you can stop once you have the case open.
Since I don't do it every day, I print out the guide and tape the screws in the appropriate place on the pics while taking it apart to make sure I get them all back in the correct location.