I cleaned my heatsink, after 6 months, and I found a lot of dust. Afterwards the CPU temps dropped by 3-5C. I used a cloth and qtips to clean it, then used an air can. Air can alone is not good enough if you're cleaning the heatsink.
A 2500k should overclock much higher. I wouldn't dare back mine down. The heart problem isn't from the CPU. What's your mobo?