Are the banshee carbs rubber isolated(rubber boot) or hard mounted to the bores? Heat and/or vibration causing mis-sync of carbs and mixture settings it seems. Carbs not 1005 in sync are impossible to tune. You can make a simple carb syncronizer from ~$7 in parts.
An old friend of mine had a banshee and was exactly as you described…awesome when running but a bitch to get to and keep at that running state.