Sounds like your talking about your torque convertor locking up. There is a mod you can do to stop it, but you don't want to use it all the time.

The torque convertor locks up to keep heat down. I've noticed mine locks up around 3k everytime, it only locks up in 3rd, and 4th.

However, when you say it surges back up. Is it coming up because you hit the gas? Or do the RPM's drop and then suddenly shoot back up?

If it isn't lockup, I'm not sure what it is.