posi additive
That's probably a big part of your problem. There's a GM TSB that's been floating around for a long time that states not to use any friction modifier with our G80s. I made that mistake in my red truck for a while and had a really similar experience. I had it drained and replaced with ACDelco standard GL5 80W-90 and ran it for a while and the G80 is working again. It's still a crappy unpredictable 10 bolt G80 but getting rid of the friction modifier brought it back to life.
I can't find the link to the actual GM TSB but I found a post on ChevyTalk where @someotherguy quoted the relevant info.
https://www.chevytalk.org/fusionbb/showtopic.php?tid/256237