At this point, I'd suspect the battery. Having 12v of stored power isn't what's needed to start a car. Having AMPERAGE sufficient enough to crank the motor is what's needed here. You can store 12v of power all day long, but if there's not enough current to rotate the crank/cam/distributor/oil pump/water pump/alternator/AC/PS (if equipped), compress the valve springs, spin the weight of the flywheel and overcome the drag of dry rings against the combustion chamber, etc... you won't be starting that motor with that battery. Cleaning the terminals is a band-aid. Usually that fixes the problem by allowing more contact for whatever amperage is available, but if it crops up again, it's not the contact that's limiting the cranking amps. You may have a dead or dying cell in your battery.

Jump it, drive to Advance/Auto Zone/Pep Boy's/wtf ever and they'll perform an electrical system diagnosis for free. They use a tool that actually puts the electrical system under load with the car running, and it finds the weak link. A voltmeter won't answer problems related to amperage, but that tool will. Once they find the problem, their diagnostic time pays for itself when you buy the part you need.

My bet is that in your case... it's the battery. :thumbup: but get that diagnostic done before you take my word for it. I've had that problem a dozen times thanks to excessive draw from audio equiment and accessories. I used to rock 1150w x 3 amps, active crossover, head unit, 10CD changer, 13 speakers in a Toyota XTra cab without a capacitor, backup battery or isolator. It ate 9 batteries in the 10 years I owned it, and each time they did pretty much what you described. I've claimed more warrantied batteries with one vehicle than most people will in a lifetime. It was usually preceded by excessive corrosion on the + terminal from the draw.