help with my engine

[rivman1243]

i get codes

21: Coolant Temperature Sensor or Circuit
31: Throttle Position Sensor or Circuit
56: EGR System
23: Air Flow Meter or Circuit

the car has a 5 pseed covertion in it
an engine with about 70k from another car in it
and a cone air filter instead of the air box




the symptoms i get with theses codes are that it will run great sometimes but most of the time it feels like there is some thing not letting enough air into the engine but then it will switch in an instant to running fine. it feels like i gave the engine a shot of nitrous when it starts to run right. other times it feels like it is missing. like a plug isint firing. and when i start it it always idles at 2500 rpms untill it warms up and then it will idle at anbout 650-750 unless it gets completely cold again. but when the idle does come down the engine surges up and down in rpms like it cant controll the idle air bypass. so i dont know where to even start with these codes.
i recenly cleaned the maf and repaired a hose that had a hole in it leading to the intake right before the throttle body. after doing that it ran a little better. about six months ago i replaced the plugs and thats about when the problem seamed to arise, shortly after that, about a month or so.
i put two cans of sea foam through a vac hose on the throttle body before i changed the plugs. i wonder if my alternator is giving out some ac voltage and causeing everything to go wacko?