I can't stand and pedal for long.. it just puts way too much stress on my knees and that was even without clipless pedals, so I doubt it's a cleat alignment issue.. and it isn't that it feels unstable, I just plain don't like it, so that's incentive enough to not do it more I do stand up and stretch every 10 minutes or so, but that's different - I'm not pedaling, except to switch legs.

I could do it non-stop as a kid and my 8yo almost never has his butt in the saddle.. but now, nope, no way.