Stalling problem 87 Blazer 5.7 TBI

OK not that it's the end of the world and I know I'll figure it out but out of all I know this has one has me wondering.
I picked up a nice 87 Blazer 5.7 TBI 121K and out of the things I have found
wrong and fixed I still have one problem.
When I put it in reverse now and then and don't give the throttle a little bump the truck stalls. Does this rarely in drive sometimes also.
New cap rotor wires and plugs. Pulled the IAC and it and the injectors are very clean. New fuel pump (It went bad) and sending unit with new sock in the correct position. EGR had a leak and was the wrong one so I put the right one on there. Also found a bad vacuum hose but that was just soft and for the heat controls. New exhaust all the way back with new cat. New O2 sensor just before I bought it.
This happens so dam fast the scanner can't catch it. I'm thinking bad knock sensor or oil pressure switch?
I know there is a loose main power wire on the starter because I lost power completely one day and crawled under and wiggled it and it has been ok for now (Will get fixed today with a new starter for the hell of it)
Any ideas guys?
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