This type of problem often happens with Campagnolo downtube shifters, causing auto-shifting (which has happened to me.) The solution is to grease the shifter threads, and completely de-grease everything else including washers !!
so ...
1. clean the handlebar center ferrule area with a grease solvent. 2. clean the stem and binder bolt at the bolt-stem interface areas. 3. make sure the threads (and ONLY the threads) are well-greased.
The first 2 ensure superior friction in the areas that should not move. The third ensures superior tightness for the handlebar pinch bolt.
When you move the stem away from the center ferrule, check to see if the previous movement (due to not enough tightness) in your stem has destroyed the reeding on the handlebar center ferrule. If this has happened, perhaps you can reposition the stem slightly to one side or another before reinstalling? Or, perhaps you can score the center ferrule or add a friction device (sandpaper?) to increase friction?
- Don Gillies
San Diego, CA