Its the Blades. Check your blades again. Also make sure you balance and track them.

Eat2muchrice’s Warped blade test
http://youtube.com/watch?v=_7PwNTXOWMs

Eat2muchrice’s Blade boil fix
http://youtube.com/watch?v=RmNv8OlG6GQ

RCFEIND’s Balancing Blades
http://www.youtube.com/watch?v=15h9G...eature=related

If that doesn't fix it then check for bent shafts and bad bearings.

Quattro1975's Bents Shaft Repair
http://www.rcuniverse.com/forum/m_6791546/tm.htm

If you still have the problem then its probably your motors. How old is it?