Runs when it wants

You will have drop the header pipes from the exhaust manifolds, and disconnect the torque converter from the exhaust pipe. Then you will be able to reach the plugs from UNDERNEATH the car.

It depends on what state your in, but there are lots of companys that make headers that are 50 state legal. If you are in CA just make sure it says that it specifically states CA legal. CA is the toughest state on smog so some headers are made just for CA smog legal. Or just pick your favorite and pay someone 100 bucks to pass you. Here is an example. As long as it doesnt eliminate your cats. You can also run like High Flow Magnaflow Cats. Those will pass smog in all 50 states.
http://store.newlevelmotorsports.com/papehe19chca14.html

replace the spark plugs with high out put spark plugs and and put a high air flow filter on it.

you have a egr system problem, most likely an oxy sensor. also check the intake sensors as they will corrode easily when let set without starting for awhile, this is why any mechanic will tell you to start a car daily and let it idle for a few minutes. (if it is a daily driver in your position)

check the mas air flow meter could be starting to go and not yet throwing a code.

the car is heated at low speed because less air flows at high speed and does not heat because the air flows more, check if the fan is turning on and check if you have good revolution,, check fuses, check the thermostat temperature, clean radiator use antifreeze coolant and finally check the pulley on the water pump with its setting correct ajustment. and finally if you work on the motor over time, verify that the ignition times are in place,

I had a similar problem, took it to get diagnosed and was instructed to give them $1100 because of a faulty reading on the distributer, rather than paying $650 for a unit I found plenty available new for under $200 I bought an OEM for $189. Now the labour can be found cheaper elsewhere. I then discovered that the module may also be at fault but I would have to replace the dizzy first, Then discovered that the Air Flow meter {afm} may need to be taken off and cleaned with a carby cleaning spray, this is a do it yourself fix costing you about $8 a can of cleaner
Unscrew the units clamps and disconnect the plug, I can't stress enough, DO NOT touch the fine wires inside, spray the carbon buildup off the inside and blow/air dry, {do not rinse with water.} reinstall the unit in the reverse of which it was removed, Alternativly you may have fuel blockage in the lines, try replacing the fuel filter {cheapest fix}
Hope this helps

if you look in the air intake boot you should see a two wire sensor stuck in it. this is the iat. That being said i dont recommend the "chip" you are referring too all it does is make computer think it is cold outside all the time so it adds more fuel to engine this is were they claim the extra power. decreased mileage and decreased engine life.a programmer works much better.

have you checked the heater core? if its clogged you wont get any heat