general problems of the 2002 Mazda Protege

Recently purchased a 2002 Mazda Protege5. Without warning, it wouldn't start. The low beam headlight burned out. The heat, stereo, headlights & power windows all worked. The car had no power and didn't even turn over. About 15 mins later, it started fine and never had a problem again until a few days ago (approx a month in between incidents). Same thing all over again, but this time the car refuses to start. What can I do? what info should I be aware of? I discovered online that this is an extremely common problem.

The car began to shutter and dropped from 50 to 25 in seconds. The check engine light (cil) starting flashing and the car began to stutter like it was going to die, I pulled to the side of the road and the car died. Started car again the cil continued to flash, it only would accelerate to 25 miles an hours even with my foot all the way down on the gas. I took it to the a mechanic and the codes were p0421, 171, 300. The mechanic did everything from engine flush, spark plugs, oil, air filters and installed a new catalytic converter but the problem persists. Its now a year later and the car is doing the exact same thing.