Intermittent loss of power after driving about 10 miles or so. Odometer 120,000

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I am experiencing power loss after approximately 10 miles or so into driving, it comes and goes intermittently and feels almost like braking. Once this starts if I come to a stop the car idles roughly and has actually stalled unless I put it in neutral. From a dead stop acceleration feels sluggish. If I park the car for a short time and drive it feels fine, until about 10 miles of driving and then the symptoms resume. Any suggestions as to what could be the cause of this?