89gta140

okay so I have a 1989 GT a 5.7 auto trans and the car runs and drives really good its only got a hundred and forty thousand miles on itthe only problem is that after it warms up it will not stay idling when the transmission is in gear and my foot on the brake. if I'm idling in park and I put it into drive the engine cuts out as soon as it goes under load. or if I'm driving and I come to a stop light the engine usually cuts out right as I stopped or a couple seconds after I suspected the icm and had it tested and it failed for maximum voltage and it also failed for voltage to the coil or something along those lines so I bought a new one and as soon as I installed it the car would barely runso I took it out put the old one back in and it went back to running the way it was previously so I got a different brand new ICM and have them test it before I installed it it passed all the tests completely and I put it in and it was the same thing as the other one it ran really rough and would barely idle.it was surging from 800 rpm all the way to almost nothing i unplugges the tPS and it idles high but really smooth. Cuts out immediately when under load. That's with the fully functional icm that makes the car bad. I haven't tried it with the original icm the one that failed the max voltage test while the tps is unplugged . My question is does anybody know why that would happen that the car runs worse with a proper icm? I'm wondering if the bad icm was masking a different problem ?. Also what are the correct steps to diagnosing t his issue? Thanks