6789mustang

I have an 89 LX 5.0-L 5-speed with 53K miles (this is not a misprint!). Car recently started missing at cruise speed (light throttle) but that's the only time it misses. Great acceleration otherwise. New plugs, wires, distributor cap and rotorbut no improvement. No check engine light and no trouble codes. Ran tank of gas through with bottle of fuel system cleaner (Techron) but no effect. Car needs new muffler (hole in one of them, all-original exhaust system)...any possible cause-and-effect here? O2-sensor issue? Have never cleaned intake or changed PCV.
[New IAC resolved stall-on-idle issue by the way (I saved the old one to clean up for a spare).]

What next to check? Suggestions?
Thanks.