We often have posts from gaming enthusiasts reporting crashes during game sessions. You have an average power supply. It is not a high quality power supplybut it should be okay while playing a game. I doubt the power supply is the problem.
Generally speaking, adding the current (amps) on each the + 12 volt rails will not equal the total current (amperage) on the +12 volt rails. The correct formula is to divide the mamixum wattage for the +12 rails by 12 volts.
The maximum wattage for the +12 volt rails can be found on the psu data label. It is usually located just below the +12 volt rail current (amps) rating on the data label.
The maximum wattage listed for the power supply and for the +12 volt rails does not mean your system will actually be using the maximum wattage. The power supply will only deliver the power necessary to operate the system. A typical pc system does not require maximum wattage during game play. The situation is similar for the current (amps) rating.