This is where I think your problem lay. I take it this MRI was done laying down? I suspect that when you are upright your odontoid (the peg-like tip of your C2 cervical vertebrae) is putting subtle pressure on your anterior brainstem. The brainstem is involved in controlling the autonomic nervous system, that’s probably causing or contributing towards your tachycardia. It would also contribute to you having difficulties running. Do you experience more, or worsening, symptoms when you look down (putting your head in flexion) ?
You will need to compare this MRI with one taken in the upright position, to be sure. If you look closely at your MRI, you can see some slight signal changes to your anterior brainstem, on the MRI where I have put the orange dots. This suggests there may be something amiss with the biomechanics of your craniocervical junction. You need to look at the angle between the yellow lines (Clivus – Axis Angle or ‘ Angle of Wackenheim’) in both positions too, which will confirm it, if thats what’s affecting you.
Also the green V shaped gap suggests that you may have some instability between the C1 and C2 vertebrae. You also need to look at the Grabb Oaks measurement, I’ve posted about this measurement before, if you want to do a search for it.
Hope some of this helps