Why is my computer going to sleep mode?

Recently my iMac has started to enter sleep mode whenever I am trying to play games or run programs that are resource hogs like photoshop and iMovie. I have tried resetting the PRAM, and the SMC. I have also cleared all the vents and made sure it is properly ventilated on all sides. This is super frustrating as this has been a great computer and I am not ready to get rid of it just yet. Any suggestions are appreciated.