No one seems to know what's wrong with my knee...?

I've been a serious runner for about 5 years with 2 full marathons under my belt. So on March 10 I went out for a quick 6 mile tempo run. Towards the end my right knee started to wobble a bit and once I finished I could barely walk. The next day I could barely bend the knee without pain on an epic scale. Over the next few days it died down enough to where I though I could go for a quick 3 miles. I was barely able to finish it. The cycle of pain for 2 days then nothing continued and even on a treadmil I can barely get a mile in. The pain is right below the patella and only when I run. I've been to 2 Dr's, had 3 X-rays, and an MRI with no real answer. After the MRI (a week ago) I was told no running for 3 weeks then come back. It's been about 6 weeks and hasn't gotten any better after lots of stretching, swimming, rest, and anti-inflamatories. It's really frustrating and all I want to do is run. Anyone have advice or experience with this?