This sounds like what I have

(from coolrunning.com)

Iliotibial Band Syndrome

Description:
Pain on the outside of your knee (not usually accompanied by swelling or locking). The pain may be sporadic and disappear with rest, only to reoccur suddenly, often at the same point in a run. Depending on the individual, this could happen at four miles, two miles or just 200 yards. The pain often goes away almost immediately after you stop running.

Likely causes:
This is an overuse injury. The iliotibial band is a band of tissue that begins at the outside of the pelvis and extends to the outside part of the knee. The band helps stabilize the knee. If it becomes too short, the band rubs too tightly on the bone of your leg and becomes irritated. The tightness is usually the result of too much strain from overtraining.

Remedy:
Patience. This one takes a while. Give yourself plenty of rest, reduce your miles and ice frequently. You can keep running, but cut your run short as soon as you begin to feel any pain. Cut way back on hill work, and be sure to run on even surfaces. Look into some deep friction massage with a physical therapist.

Try some leg-raise exercises to strengthen your hips and be conscientious about the iliotibial band stretch. You might supplement that stretch with this one, doing it gently but often:

To stretch the IT band of your right leg, stand with your left side facing the wall. Cross your right leg behind your left, while putting your left hand against the wall. Put your weight on the right leg and lean against the wall by pushing your right hip away from the wall. Be sure that your right foot is parallel to the wall during the stretch. You should be able to feel the stretch in your hip and down the IT band (in this case, along the right side of your right leg). Hold for five seconds and do this ten times. For the left leg, do as above, but stand with your right side facing the wall, and put your left leg behind your right.

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