AMD Athlon 64 X2 6400+ stock cooler issue

Hi,

I am having strange CPU fan problem. I am using AMD Athlon 64 x 2 6400+ with stock cooler. Recently I've been monitoring my system with Everest and I noticed that my fan is spinning at low rates when on load (especially when playing games). For example when I play Dead space (puts GPU/CPU on max load) my CPU temp reaches 67 Celsius and fan spins at a rate about 3300 RPM although it is capable of doing 6000 RPM. Same thing happens when stressing with prime95 just the temperature gets higher (about 73 Celsius). AMD manual says that this CPU with stock cooler reaches max 63 Celsius. My PC case is old so I leave it open for better cooling. CPU fan just won’t bother to go faster until I close the case (in other words, until I make hotter inside my PC case). Any ideas?

I'll keep in mind, thanks