09 Sierra

I have a 2009 Sierra. I noticed the voltage dropping to a concerning voltage for some time now. I changed the alternator and still had the same trouble. I drove down to Florida from Connecticut and when the voltage dropped below 11 I would put the truck in neutral and turn off the key and then restart it and it would start charging again. Sometimes it would charge for a couple hours before dropping that low and sometimes it would only take about 10 or 15 minutes. Because it never really happened until after about 45 minutes the dealer never really was able to find the trouble. I brought it to a local shop and they identified a problem and said it was the ECU. I had that changed out at the dealer because he didn't have the equipment to flash the chip and now it is worse than it was before. Truck only has $65,000 miles on it. This is not the fuel saving programming that they put in to it this is a problem. Once it stops charging it never starts again until I turn off the truck and turn it back on which is telling me it's a computer issue. Any help would be immensely appreciated