I recently installed a new idle air control valve. The car was starting to try to find the normal idle speed but I noticed that it started to momentarily loose power. It suddenly shut off but turned right on. As I drove the highway it started again. I made it about 20 miles and it stopped. Now it will start very momentarily but will not idle. I’ve checked the main relay and replaced the fuel pump.
Most common reason for this issue would be a vacuum leak. A quick test is to simply press down on the accelerator a tiny bit and start the engine. If it stays running, then you probably have a vacuum leak. And your old IAC was probably just fine.