Gears slipping?!

[JustWannaSpeed]

As I am a speed-head, I like to modify the engine config for cars to give them tons of power to haul some serious ass, but sometimes when I add power: It'll be at say, 2,500 RPM, I'll hit the accelerator, and the RPM's will fly up and stay in the same spot, but not redline. It almost seems like a slipping clutch or gears, but I'm not sure. Has anybody ran into this before? Thanks for the help!