In all likelihood the thermal paste has dried up. You will want to clean off the thermal paste from both CPU and the heatsink as soon as possible. Stressing a CPU (like playing games) can cause temps to rise by 25C so when playing games your CPU might be running at 95C or higher.
I recommend Arctic Silver 5 or IC Diamond thermal compound. Excessive heat can cause the CPU to fail.
+1^ Replace your thermal compound. I found that Arctic MX4 is also a good TIM with the added benefit of not being electrically conductive. Be sure to clean the old stuff off the CPU and cooler before putting on new. I use Arctic cleaner, but isopropyl alcohol works good too.