Sluggish

I have a 2009 challenger rt automatic.  It has 57k miles on
it now. I bought it with 49k.  I have an hour drive to work
mon-fri so I know the car pretty well. 2 weeks ago the car
started acting really sluggish. When you gas it gas, instead
instead of downshifting like it usually did, it stayed in same
gear and gradually gained speed. This usually happened
when on highway going at speeds 60-80 .Oil is fine. No
check engine light. Nothing. I have taken it to 2 dealerships
to fix it, one of which being a dodge dealership, but no one
seems to be able to find a problem.  Like I said I drive the
car everyday, i know when there is a difference and loss of
power. I'm planning on taking the car back to the dealership
Friday and I need some suggestions on what the possible
problem may be so I can have them check everything out..
has anyone ever had this problem or one similar.. I'm not
totally knowledgeable about this so any feedback would be
appreciated!