The outside of my left knee hurts only during running?

I was avid athlete, having competed in many triathlons, marathons, etc. My knee suddenly started hurting in middle of half marathon. It was a dull pain on the left side of my left knee. It quickly went from dull to sharp and i had to walk/limp the last 5 miles. I haven't been able to run since that day 4 years ago. One doctor told me I have arthritis (as shown on mri). Another doctor did exploratory surgery and said no arthritis and couldn't figure out what was wrong. Then acupuncturist told me my IT band was tight and not aligning correctly. he prescribed deep massage (which was excruciating, but I'll do anything to run again.) What next? I've tried orthotics, new shoes, a knee brace, a band around the top of knee. I take karate extensively and with zero knee pain. Running is my passion, it's my therapy. In my heart I feel like it's something mechanical that's wrong that can be fixed, but no one can tell me what it is. Any ideas? Like i said, mri shows very beginning of arthritis. Surgeron cleaned it and said it's fine. I can jog for about a mile before my knee hurts. It's impossible to run through this pain. If I walk for a minute it recovers quickly but as soon as I start to run, instant pain. It absolutlely only bothers me when I run. Unfortunately, that is what I want to do most. HELP PLEASE! or give me a direction to go.