1999 Dodge Durango

I recently had a tune-up done on my 1999 durango. The check engine light came on afterwards.I had it tested at the autoparts store and it threw a P0505 code. I bought a IAC motor and put it in and the light went off. Two hours later I started it up and engine light came back on. I took it to my local dodge dealership and they replaced the IAC motor (AGAIN) and reset the PCM. It was same thing I replaced and when I asked why the one I installed didnt work they told me that aftermarket parts dont work on Chryslers, first I ever heard of! And they charged me 85.00 for the part itself and 135.00 in labor. I got the old part back and it looks exactly like the one I bought at the auto parts store. They test drove it and said the engine light didnt come back on. When I went to leave the dealership the engine light came back on, I took it back (didnt even get out of their lot) dodge looked it over again and I was told that the computer is probably bad. They wanted 1200 dollars to replace it. They quoted me 700-800 for the computer and the rest was labor. I think that is alot of money to spend for it to "Probably" be bad. I went to the local auto parts store and to see what it would run. They quoted me about 325.00 for the computer, but the guy there said it has to be programmed before it was to be installed. Is this true? And if so how do they program it? I really dont want the dealership mechanic to look at it again because he acted like it was a big problem and he didnt want to deal with it. I think he was a lil mad because I brought it back!