There are several threads on AF where members have installed Duralast secondary ignition parts on their 4.3 engines, (do a search). They have experienced problems either right away or shortly after the parts were installed. This includes
all GM vehicles with the 4.3 "W" & "X" engines, not just Blazer & Jimmy. If you were to put the vehicle on a scope, you'd see immediately if the problem was caused by a fault in secondary ignition. But, since a scope is not a piece of equipment found in the average DIYer's garage, it's another important reason to use quality parts. I'm not saying your parts are faulty, but it's a possibility. I would suggest starting with an AC Delco cap and rotor, and see if you can get the fuel pressure and leakdown results. Might not be a bad idea to add a fuel pressure gauge to your tool box
They're like $20