XJ Rookie

I have been having this problem for about 6 months and have already thrown a few parts at it for lack of a diagnosis.

MAP sensor and vacuum line replaced as well a full tune up including fuel filter (which was my first suspect.) Also had to replace clogged cat and cracked exhaust manifold.
Most recently replaced alternator to clear codes 12 (battery connection) and 41 (alternator field circut.)

Now to the symptoms. About 15 to 20 minutes into my drive the check engine light kicks on(code 51-lean condition) and usually moments later I feel a somewhat violent jerk. The engine has never died or really even come close. Just that split second stutter. Always at cruise. Not really associated with the engine under load or acceleration; just maintaining freeway speeds. And it only happens on the freeway. Always 50mph +.

When this happens I pull over. Wait 5 to 10 minutes restart the jeep. And I'm on my way for usually another 5 to 10 minutes.

I'm pretty stumped. My only thoughts are maybe a small vacuum leak or the fuel pump. But I need some outside insight so I don't run myself in circles.

Anything you guys can think of I'm all ears