I don,t know what engine you have but on my dodge stratus 2.4 fuel injected I had the similar problem with stalling and it struggled at higher speeds on the freeway. I suspected it was fuel pressure,or carbon in throttle body, or perhaps fuel injectors , or coil, or Idle air control motor(valve some call it) also timing and plugs and wires were all suspects. Plugged cat was also on the list along with a vacuum leak. It was none of these! Well after 4 weeks of messing with this engine I finally pulled the egr valve off,New just last year,and mind you i had no check engine lights for my problem. I pulled the transducer valve apart and found it had melted the diaphragm. Also i noticed the line going to it ,which is exhaust gasses, and carbon had almost tottally plugged it. I did replace my IAC for the second time in a month..the first one was not functioning at all, got it from advanced auto. this car now is running beautiful after replacing egr.