Turtle Ted is right on here. Heed his advice. The tension on the spool should be just enough to keep it from unwinding. If you're flattening the flux core wire there's too much tension on the drive roller.
The inner lining could be wearing out and/or rusty/dirty. I have an option on my Hobart that applies a lubricant to the wire just as it enters the liner. It was a Hobart option so I don't know if there's such a thing for your machine.
Above all the tips MUST be clean and free of slag; a problem with flux core wire and a good reason to look at a gas setup. They burn much cleaner.
Good luck. Let us know if these suggestions help.
Just thought of a question: If MIG is metal/inert gas but the machine is not setup for gas and only uses flux core wire, is it still a MIG welder? I'd think you'd call it an MF