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Missfire under load

What does it mean when your car misses under load. Going uphill or trying to pass, or sudden acceleration , like entering a highway? I was reading and I do not think is trans since that was rebuilt 3 years ago . It Is Definably a MISSFIRE! I am wondering is it fuel or spark? My Haynes book suggest a bad cylinder. OR!! can it be a bad Catalytic Converter??? 95 Ford Escort, Cat Conv connected directly to exhaust manifold, and heat shield recently rusted away and fell off!! In the owner’s manual it says that the fuel filter is a lifetime filter and does not need replacing. 147K miles? Platinum plugs. Maybe wires? Symptoms intermittent to weather,does that mean anything? Used to be to let the car warmup, but happened bad today after an hour of highway driving in nice weather. It idles fine but misses when I put it in gear, until I get a good momentum going. When I slow down at highway speeds, like going throgh a toll booth, and then try to accelerate back to highway speed it misses very badly.
Thank you All in Advance!

Fuel filter, (its had its “lifetime”) new plugs, new wires …Try that…

Caddyman, I thought the same, old saying…" when you see hoof prints in the ground in America, think horses, not Zebras" . In other words, stick to the obvious basics. Thanks!

Hopefully it’s what Caddyman says. If not, next suspect is plugged cat.

Let us know what happens after the plugs and wires. Also, have it checked for codes.