BSoD BCCode 3b and 7e- random but mainly games

Hello, sometimes I'm having random BSoDs. Most common BCCode's are 3b and 7e. BlueScreenViewer says that in 90% cases ntoskrnl.exe is faulty. BSoD' happens mostly when I play but also in some strange situations like closing the game, or pressing play on flash player movies... I wonder if it's GPU software or hardware problem... I attach minidump files. Thank you for any hints!