97 Chevy Astro Intermittent miss during cruise

1997 Chevy Astro Van 4.3 code W, automatic with 154,000 miles.
I have replaced fuel pump, fuel filter, spark plugs, wires, cap, rotor, and coil. Fuel injectors were cleaned with OTC type cleaner.

Van will have an occassional miss ONLY at cruise. Engine idles good. Miss dissappears during hard accelleration. OBD II scanner shows both long term and short term fuel trims to be balanced and generally within +/- 10%. Only code is P0420 -cat. Converter below threshold. Fuel pressure is 60 psi with key on engine off and 55 psi with engine running. I suspect that I may have an injector going bad but that is an expensive "guess" if I'm not right. Any suggestions would be appreciated.