1. Clean/replace case/HSF fans. (120mm Yate-Loon fans are around $5)
2. Invest in fan filters (pantyhose works great BTW).
3. Move PC off the floor!
4. Re-apply TIM, and Re-seat the HSF. (are you using the stock HSF? if so look into an aftermarket HSF. The CM Hyper 212+ is $26) *CPU*
5. Is there a high CFM intake/exaust fan in the case? (intake from front, and side. exaust top, and rear)
6. Try a new case (look at the Xigmatek Asgard $30 + $8 S&H).