2000 Chevy Impala open/closed loop

I have been working on my daughters 2000 Impala, and it runs fine at an idle, or at speeds under 50 mph. When you attempt to get on the freeway, or even on a 55mph road, the car soon begins to cough & choke. I have checked the fuel pressure, and it is within in specs for the KOEO and at idle.

I borrowed a scanner, and hooked it up while driving the car. The only thing that I have found is that when the car begins to act up, it switches from Closed Loop to Open Loop. I put the car in neutral, and let it idle, it comes right back to Closed Loop, and the car drives normal for a bit. But, if I try to continue to drive the car, it stays in Open Loop, and continues to choke & misfire.

The car has new plugs & wires, a new catalytic converter, and the fuel pump was replaced a few months ago. I have been told that it may be the ECM. Is there a way that I can be more sure, before I start spend her money on a wild goose chase?

Thanks,

Matthew