Fastball rotation problem

Question
Hi. I throw a four seam fastball around 70.  But when I go to pitching instruction, the coach says I can gain 5 mph on it if i fix the spin. I am a righty and my ball moves quite a bit in to a lefty. I like this but i would also like a 75 mph fastball.  Any ideas on drills?

I have tried the towel drill and i can get it to snap like a use to be able too.  And i cant get my hand behind it at full speed even after doing it half speed.

Thanks

Answer
I think you're saying that you have a natural "cut" on your fastball and you want to get rid of it.
There's no guarantee you will gain 5 mph, first off. I would check your grip first. try throwing different fastballs and seeing what each of them do.
If nothing works, then embrace the fact that you can throw a natural Cutter. watch Mariano Rivera and learn to use it like he does. gaining 5 mph but losing movement isn't that great of a trade off.
Jon