This has been a weird issue with my MacBook for the past couple months or so. Whenever I'm doing pretty much anything, like Safari, Dashboard, or iTunes (not even with all of them open at the same time), the screen starts slowly refreshing from top to bottom. After that, everything starts lagging and the fans start blasting. I've already looked in Activity Monitor and nothing in the background is sucking up any CPU, and usually I'll have a lot of free RAM available. Right now I have 1.24 GB out of 2.00 GB free and just scrolling up and down in Safari seems like a chore. The fan is running normal but the temp is hovering around 130-135F (54-57C) right now, which seems a bit high for just using Safari 3. I've done pretty much everything imaginable to fix it: I repaired permissions, used OnyX to clean out everything, moved ALL my preferences (root and user) into different folders so I could start off with fresh prefs, and I've even created a new user, but none of those has seemed to work. I've even blasted the dust out of the fan vent. There wasn't that much dust, but that didn't help either. It's really confusing as to whether it's a software or a hardware problem because if I start up in Windows, the fans still blast whenever I do stuff but it doesn't lag like it does in OS X. But when I restart to go back to OS X, it starts lagging right from the gray Apple startup screen. Has anyone had this happen before? I'm thinking maybe I should go to the Genius Bar to see if they could tell me what's going on, but I'm out of warranty.