The RPM's always jump when you let off the gas. That is a "safety feature" that is programmed into the ECU. It adds fuel when it sees a sudden drop in load or throttle position to prevent knocking. Nothing to worry about.
I just put a blitz turbo timer in and now when i am running it hard i get a big rpm spike in between shifts. I had just reset the ecu also. I checked all the hoses i messed with durring the instal and they are all hooked up ok. This rpm jump is like 500-800 rpm when i am running full boost. Could this still be the safety feature cosmo spoke of?