Why does your tattoo machine coils get tired?

It could be a combination of things. In all likelihood, you have bad connections to your contact points or the wires leading from your coils might be kinked or frayed, leading to a bad electrical circuit. The capacitor to your coils might need to be replaced as well. You should look into getting frames made of pure copper as it has the best electrical conductivity of any metal that you could hope to use and therefore the flow of the circuit from your power supply will be nice, smooth and uninterrupted. You should also convert your machine from a standard clip-cord configuration, to receive an RCA jack-cord. When you convert your machine to receive power in this manner as opposed to the old fashioned clip-cord, it makes the receive of the electricity more solid and since it is firmly fixed in place around it's nozzling, the cord does not sway from side to side when you are working so the flow of power is never broken. You might also want to look into getting a digitally powered power supply. They allow you to precisely measure your flow of power within a great amount of accuracy as opposed to just sort of guessing it out with your standard DC power supply.