What brand of "universal" O2 sensor? By "near your cat" do you mean before or after it?
Ignition problems rear their heads at high-load and high-RPM, not idle. Same goes for fuel problems. They're always more prevalent when you're flooring it.
The alternator could be the culprit. Bad alternator output can cause the ECU to behave erratically because of bad input signals. Have it tested, it won't cost you a dime.
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