04 Frontier cruise control/tranny

04 crewcab, longbed, automatic, 3.3 non-s/c. Cruise will not hold speed. Will lose speed and upshift on the slightest incline, then when going downhill after, will not downshift to o/d. Stays in 3rd, RPM's about 3500 at 65 mph, then I have to disengage the cruise to get it to shift back into overdrive. It seems to work fine at 60 mph, but go above and try to set at 65 or 70, then the problem shows up. Dealer finds no codes or problems. Also, anyone know the expected engine RPM's at 60 mph? I'm thinking I jave tranny problems, but am getting no help from dealer.

Have your catalytic converters checked. Just a thought.