There once was a problem with the pressurised fuel line in some models.They rubbed on the inside of the tank,wore holes in them and consequently lost most of the fuel pressure to the engine.
The bikes would run ok with a full tank;once the level dropped,fuel would leak out of these holes and the bike would run erratically.When your fuel level drops to half capacity,listen for hissing or spurting coming from inside your tank.
I cant recall which year bikes were affected,but try looking in the self help section.
Just a thought.