Battery seems to provide intermittent power

Mitsubishi replace my battery, plugs, sparks, and timing belt about 6 months ago. Suddenly, Monday morning, the vehicle would not start. There were no dashboard lights at all and the starter would not even click. I jumped it and drove it for about 20 minutes, parked it, and turned it off. I turned the key back to start the vehicle and there were no dashboard lights and the starter would not click again. I jumped it again, took it to O’reilly’s and they checked the battery. I parked and turned off the engine and it once again exhibited the same dead symptoms. They said it was bad and I need to replace it. I went out to the truck and heard the door chimes. I turned the key and it started! I have no idea what is happening. Mitsu tested the battery and said that it is good. They said it was probably my aftermarket head unit draining the battery. This just seems ridiculous as no head unit can INSTATANOEUSLY drain a car battery, I believe the battery would explode. Please help, I'm trying to trade it in today or tomorrow!

I know this is a month old, but my bet is bad/dirty cables/terminals. Clean them up, tighten them down. Corrosion or lose cables can cause this problem. Hope you've solved it by now.

It's amazing that 2 dealerships couldn't see that the terminals were horribly corroded. The day that I sold it, I thought, hmm, could the dealers have missed corroded terminals? So I checked and wow, they did. Cleaned them up and everything was fine. What tools.