For some it helps to go even faster first instead of slower. Instead of going from 170 up to 180, try setting the metronome up to 200 or 220. Let yourself listen for a second and imagine what it would sound like and then try it. Of course you won't get it and it will probably be sloppy. But once your done trying it at the extreme and way beyond your reach, decrease the metronome down to where your stuck and then try again. Suddenly the tempo you were stuck at doesn't seem so fast. This works for some people when they plateau, so try it.