i would have used all delco parts on the tuneup. That may be something you want to try, seams these old chevys like delco parts best.
also add a can of seaform to the next couple of tanks of fuel this will help clean the fuel system.
I had my 2000 model 2500 series with 350 engine tuned up at 90K because I planned to make a trip pulling our travel trailer, but the old truck was running fine before the tuneup 90K miles.
Truck still runs great.
might also check your shocks when you get the running issue taken care of.
my shocks at 90K were completely dead, new shocks made a difference in ride