2000 mazda protege o/d light flashing & check engine?

[Nik]

bought the car with 35k miles and check engine light came on not long after. took it to the shop, siad it was catalytic converter so i got that replaced.light came off, but about a week later it came back on, so i took it back to the shop and they said it was the air intake manifold so replaced that and the light went off. a couple months later i started hearing a rattling sound coming from the car when i drive up until about 2000 rpms. not long after,my overdrive light started blinking and my check engine light came back on, and my transmission seems to be slipping. how could this be happening at only 50k miles. please any advice would help