1989 chevrolet astro 4.3L TBI 167,000

I'm having a problem where sometimes the van runs great and at other times it misfires, backfires a little, loses all power and sometimes dies. I changed the distributor cap & rotor button. I had one place say fuel pump & another say throttle position sensor. I am at a loss and don't want to pay for a wild goose chase on what it is. the check engine light never comes on.