How to deal with an inconsistent / noisy power signal?
The 12V → 5V → 12V is going to take up quite a bit of space and may waste power. Might be worth looking into other options first such as:


a 12V regulator (78xx) 
a reverse biased zener diode. These are simple, cheap and fairly effective. If your circuit needs to be very temperature stable you may want to look at placing two zener diodes in series, one forward active and one reverse biased. Generally speaking, the larger the nominal zener voltage, the larger the temperature coefficient
if you have a digital circuit or any circuit with lots of switching you’re going to need a  capacitor (or a few) to help counteract parasitic inductance (check out dc bus capacitors). But capacitors in general are a good idea for stabilising your input voltage, especially when you turn the circuit on
if EMI is an issue try checking out using twisted pairs or coaxial cables