Adjusting a triple chainring with a 9-spd cassette -- shimano. It runs smoothly through all gears and up & down chainring EXCEPT on the middle chainring and small cog -- the chain gets caught on inside of the chainring, rubbing and literally picking-up chain on inner connectors causing chain to pop-offf chain ring and back down again.

I have done the following and cannot get the problem to stop:
1. adjusted front derailleur soit runs smoothly between small, middle, and larger ring
2. adjusted cable tension for front derailleur
3. tightened BB as much as I can turn

Anyone know what is going on and what I need to do?! THANKS!