What is causing my acceleration problem on my Infiniti i30 after replacing the IAC and ECM?

Ok, so a few months ago, My Infiniti i30 had a problem where it would stall if I was idle at a stop light for too long. I had 2 mechanics verify that it was both the IAC valve and ECM that went bad. Turns out, this is a very common problem amongst 2001 Infiniti i30s between the 80K and 100K mileage range. My mechanic replaced my IAC with a brand new one, and found a refurbished ECM that was matched by VIN and serial number. He replaced both, the ECM codes and all that tech stuff was done to make sure the new ECM was registering the new IAC and the car was driving fine until it got to the freeway. When you step on the gas, the car will not accelerate. It bogs down and won't go fast at all. I don;t know what the exact RPM was but the car was definitely not speeding up and this wasn't a problem before the IAC and ECM were replaced. What's causing this?