Runner's Forum: New Sole F80 Treadmill hurts my knees when 10 yr old prform didn't.

I just bought a new treadmill.  I thought I was getting a good one.  I did my research ect.  I have now run on this treadmill twice and both times my knees ache and my lower back hurts.  I notice that it feels kind of like running outside when running on this. I never noticed my old treadmill feeling springy or anything, but this one feels hard!  I am wondering if this is just the way new treadmills are?  I am thinking of going to the gym to see what theirs feels like.  I am disappointed though becuase I don't even know what treadmill to look at next.  I read that Sole treadmills are the best for the money and I spent 1500.00 on this one.  Any ideas???