starting and stalling problem

I have a 2012 Buick Enclave. Recently developed a problem with stalling. I can start it up, but once it warms up it stalls. Once it stalls I can hear and feel the powertrain relay clicking on and off, along with the throttle body clicking as it opens and closes. I thought it was related to the TSB on the under the hood fuse box with the number 51 terminal being corroded, but I replaced the fuse box, and even wired in a external powertrain relay, and it didn't solve my problem. Tested and charged the battery and it is good. Replaced ignition switch too. I keep getting a P1682 ignition code, and a bunch of U codes, losing communication with the PCM, BCM, and ABS module. When this problem occurs, I am unable to communicate with my scan tool. I noticed when the problem occurs, my powertrain relay control, which should be grounding the powertrain control relay circuit, loses its ground and I have about 5 volts on that wire when it should be grounded. Additionally, the powertrain relay power wire that should have battery voltage, starts fluctuating voltage around 7 to 8 volts. From what I can tell, whatever tells the PCM to ground the powertrain relay control is failing, but I can't tell what tells the PCM to ground that circuit.