You may have been lied to about how much capacity your power supply can actually output. Not too long ago there were a ton of fly by night companies that would sell power supplies that said they were rated for 600w but were lucky to output 300w. There still are companies that make shoddy power supplies that can't distribute what they are rated for.
Also, power supplies do burn out over time. As the capacitors age, the power supply isn't able to put out as much wattage as it once could. If the quality was questionable a few years ago, it's quality is worse now than it ever was before and it will continue to go downhill.
The true test is to add up how much output is on the +12v rail. Volts X Amps = Watts
A true 500w power supply will come with a 6-pin or even an 6+2pin PCIe power connector. When you said you connected it through the Molex connector adapter, my suspicions as to if you have a capable power supply went into doubt. I don't think you do and getting a decent power supply will likely be the remedy. A cheap but decent power supply is the Corsair CX550. You can also look at the Seasonic M12II 520w which is an alright power supply unit.
The power supply is probably the most underrated component because it doesn't actually increase a computers performance. On the flip side, it does keep your PC from frying or dying. A good power supply does a better job at giving components a more stable feed of power which will result in your computer lasting longer. If you go read reviews of power supplies like the EVGA G2 series or another power supply that's good, you will see that what I'm saying is true.