After riding bike for approximately an hour one or the other of my outer upper thighs begins to experience pain from the hip to about the middle of the thigh. The severity is directly related to the effort used to pedal. If I shift my weight on the saddle away from the painful thigh the pain is relieved. Also extending the affected leg straight as I’m riding gives temporary relief. No other discomfort is noted. Seat height and position appears correct. Could the pain I described be the result of an incorrectly sized saddle or perhaps something else?
Are you maybe rocking sideways in the saddle? Could be because the saddle is too high, could be because you have insufficient hip stability. Weak (or underutilized) abductor muscles of the hip (especially the Gluteus Medius and Minimus) are a common ailment. But these things are hard to pin-point. I once had Iliotibial Band Syndrome (ITBS) along the outside of the thigh down to the knee from cycling and it was really due to weak Gluteus muscles.
Can you film yourself from behind and from the side at a high frame rate? It could give you some clues what’s happening.