bokomba

I have a 92 LS and it kept stalling / it gave engine code 12 which was right rpm signal / I replaced both cam sensors / the car was fine for a while but now I'm back to square one and this time it gives codes 12 & 13 which are both rpm signal codes / it will stall while idling and its hard to start up / should I mention this is a replacement japanese engine and trans? and I also did timing belt,water pump and major tune up / did job on my own and made sure plugged all wire connectors securely / just trying to narrow it down /does some one know what exactly these codes mean and what the problem may be ?----