Serious Mazda RX-8 Question? Loss of Power above 5k RPM?

I have an 04 Mazda RX-8 that I just purchased about 2 weeks ago. The vehicle has a new engine installed by mazda with only 20k miles as well as a brand new clutch. The last couple days I have noticed a loss of power past 4500 rpms like something is pulling it back. When I hit my power band at around 5k the vehicle loses power and starts to jerk. I don't have this problem in 1st and second gear. Only 3rd to 6th. Please help me figure out what's going on.. I only use the highest quality non-ethonol gasoline and I check and fill the oil every other fillup.