My iMac Keeps freezing When Gaming

i normally turn on my computer and log on and i open up skype and minecraft and 30 mins later after playing it my imac will start becoming unresponsive and it will shut down and once it boots it says my imac shut down because of a problem i did a check on it with the recovery mode thingy forgot the name to it and it said nothing was wrong so again i booted up the computer again and play minecraft and it freezes again idk whats going on... i have 4gb of memory and a 1tb HDD hard drive