I need help!!!! please

Hello, I bought a 99 cougar automatic. I was excited of course but things started going downhill after 4 months. The car was idling very low and it wanted to turn off and sometimes it would so i replaced the air idle valve. well i just started my job so i didnt have the money so my friend put a valve from a newer model of a cougar on my car...well then my car was idling very high....when i got paid i replaced the valve with the right one. well my car worked fine then...my brakes were very hard to push and my power steering was acting up. I took my car to midas and my abs light came on. So they looked at it and noticed my belt was sliding off so they had to replace the pulley. well they fixed that and i drove my car for a week and everything was fine THEN....all of a sudden now its idling crazy and when i stop at a light or whatever the car will rock back and forth....they thought it was the EGR system but its not that. So they think it can be the intake manifold!!! Can anyone help me!! im soo stressed out i just want to get my car fixed! please help!! :uhoh: