One more thing you can try is adjusting the pulleys with a slight bias towards the bigger cogs. The chain has a tougher path climbing up to bigger cogs than dropping down to smaller, so the trick would be to line the pulleys up directly under the 4th/middle gear, then - as you're seeing it from behind the bike - adjust the cable so the derailer moves the pulleys slightly left off-center. I set all my derailer bikes up this way.