Were you doing something in you case and bumped the heatsink. This could cause a small air bubble or uneaveness in the thermal grease and increase temps. These increases can be pretty dramatic if the heatsink is not seating 100% flat any more. Bumping the heatsink is very bad also if you are using a thermal pad. Temps can go up quite a bit.
Dust in the heatsink can cause truble as could a fan slowing down or room temps changing. I also noticed that the temps on my Duron would spick some times, but I think that was caused by my old power flucuating in power output. After the power supply died, it never did it again.
AMD Phenom II x4 945 3Ghz | ASUS M4A77TD | 2X WD 1TB SATA 2 hard drive | 2x2GB Corsair XMS3 | nVidia GeForce 8800 GTS | ATI TV Wonder Theater Pro 550 | Antec P-160 case | Antec 650w Earth Watts | LG Blu-ray Super Drive | LG DVD RW | Windows 7 Pro