It only misses between 45-50 mph, I suspect the transmission is jumping in and out of overdrive.
Take it to a professional and have the valve lash adjusted. Also replace the spark plugs.
Check/replace the spark plugs and wires. If you're not going to replace the plugs make sure the gap is correct (should be either .040 or .060) and make sure the plugs are clean of oil or gas. If it still backfires after you replace the plugs/wires check the distributor and the timing.
Assuming the plugs and wires have already been replaced you may have a bad fuel injector.
whenever you change the plugs it is a must that you change the coil packs and wires. for some reason the plugs never seem to fire properly with the old wires and packs. i replace everything about every 40,000 miles
the problem will be one or more coils and plugs needs replacing
I would replace the spark plugs after 50K, the wires and plugs again at 100K.
Sounds to me like you need to replace the spark plugs, spark plug wires and rotor. If the condition still persists, replace the distributor cap.
No you do not need to replace any plugs at all when replacing wires.
If you gap all the new plugs first and then replace the plugs and wires one at a time you won't have to worry about messing up the firing order. (This will happen if you pull all the plug wires at once and don't replace them in the same locations.)
how to replace spark plugs wires 1992 buick century v 6 3300 stationwagon
With a socket for removing plugs. The wires should just slip off and need to be replaced with the exact same length.
Does it miss on acceleration or at idle? If on accel I would be looking at the plugs and wires