You may have a flywheel problem, I ran in to this the other day.
I got a call from my mother in law, she described having the same problem on her subaru. In higher gears & faster speeds it would go away. Eventualy she got stuck at an intersection & couldnt take off in first gear.
being an hour away, I couldnt diagnose the problem over the phone.
She had it towed to the dealer, they found a cracked flywheel.
