Why does my 1995 Riviera miss in overdrive going down the road

PROBLEM: Intermittent engine misfire under maximum load (overdrive gear with torque converter locked) .. ANSWER: Spark plug wire(s) breaking down under a specific load that only occurs while in that particular (overdrive) gear and engine rpm.. Shifting into 3rd gear 'changes' rpm, lightens engine load and 'temporarily' alleviates misfire .. Get a tune-up .. New spark plugs and spark plug wires at minimum!.. Suggest ACDelco parts or at least 'quality' equivalent replacement parts... Many auto parts stores sell ACDelco... Due to the difficulty of *firewall side* parts replacement, it's not DIY friendly.. May need qualified hands-on help.. Happy motoring!!