I"m new and got questions!

I've had my Rx8 for awhile and I'm having issues with it, issues that hadn't come up in the past 6 months. It seems whenever I drive 4th gear gets very little gas and revs, it won't go above 3,000rpms unless I'm on the highway and going 70mph+ and it has to do to sustain the speed. Whenever I go up a hill(even a small one) 4th gear kinda fails and it switches down to 3rd gear sometimes even 2nd. It happens even if I don't step on the peddle any harder then regular acceleration. Was wondering what could be wrong, my check engine light it is on and is giving me numbers about the computer? When I just got the CAT converter replaced because it wasn't breathing correctly. Any suggestions would be fantastic!