eMedicine article on Iliotibial Band Syndrome - sounds most likely
eMedicine article on PCL injuries
eMedicine article on plical irritation
eMedicine article on patellofemoral syndrome - most common anterior knee pain cause ever
These are a couple of thoughts... I'm not an expert by any stretch, but I'd be surprised if it were sciatic pain. Sciatica doesn't feel better after "warming up". Some people feel better after stretching, sure, but hopping on a bike for a bit? No. Typical presentations of sciatic pain are shooting pains down the legs, butt pain, etc. Not isolated knee pain.
I'd be getting second opinions, and from as swanky a hospital as I could find. Where are you located? I might be able to point you in the direction of awesome orthopaedic doctors.