Why is my car misfiring on and off?

It usually takes 15 or so minutes to misfire. Sometimes it never misfires for 300 miles. I've worked on this for a year now trying to find the cause. But the key to this puzzle is it's very intermittent but can be very stubborn. Usually starts at a red light then goes away once I get up to 35mph+. Or it... show more It usually takes 15 or so minutes to misfire. Sometimes it never misfires for 300 miles. I've worked on this for a year now trying to find the cause. But the key to this puzzle is it's very intermittent but can be very stubborn. Usually starts at a red light then goes away once I get up to 35mph+. Or it just stays at any speed and goes away at a red light. It's so random. Nothing seems to control the misfire. It just misfires whenever it wants. But always starts up fine. And there's no other issues going on with the vehicle. My emissions test were great. The misfire feels like one cylinder or maybe two on the bad occasion. It's an i6 4.0L 93 jeep.
My biggest question is if something's broken or worn out why doesn't it misfire all the time. What would cause a random intermittent misfire and also cause it to drive perfectly fine 70 percent of the time?
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