Had mri/xray on knee doc said it was fine , but it pulses and has pain in knee, feels unstable?

Knee pain: Most knee pain won't show findings on X-ray (primarily looks for advanced arthritis or something big like a fracture). MRI is excellent at showing ligament and cartilage injury, but it's used primarily as a pre-surgical evaluation for pain that does not respond to physical therapy, bracing (soft knee sleeve) and anti-inflammatories which all take time but work well for most non-surgical knee pain

Hard to say: It is good to know your MRI looks fine as that would show the major ligaments in the knee and whether or not there is a problem there. Vascular or neurological causes are possible although fairly rare; it is also possible the MRI was miss read or missed something. Don't hesitate to get a second opinion.