Self Treatment - a hard thing to do

Need some assistance:  I am a triathlete who is training for my first marathon in December.  In the past 6 weeks I have had an issue with my left knee feeling like it makes a loop during the mid-swing portion of the gait cycle (The period between maximum knee flexion and the forward movement of the tibia (shin bone) to a vertical position).  This loop is from the neutral position then lateral.  I notice it more on long runs (14 - 18miles).  There is discomfort to pain just proximal to the lateral epicondyle of the femur.  Have any of you had clients that presented this type of knee issue?  I am wondering if I am having a muscle tighten mid-run and pull laterally at the knee or is a muscle fatiguing and allowing the knee to become lax?    It is frustrating to not be able to 'Fix' yourself, so your input will be greatly appreciated.