GP1800R with Solas 13/20. Prop vibration out of the hole

I have a 2020 GP1800R. I flashed the ECU to a Riva Stage 1 and changed the prop to a Solas 13/20 (I’m still waiting on the rest of the parts to arrive next week to complete the Stage 1 upgrade).
Now, when I hit the throttle from a dead start I get a vibration from the rear of the ski for about 1/2 a second and the RPM’s seems to go really high, and it doesn’t seem to be hooking up very well. After the 1/2 second it hooks up very solid and the vibration goes away. I don’t get this vibration any other time, only when I hit full throttle from a dead stop.
Any ideas of where I should start looking?
Thanks, Mike D.

Cavitation related to generating additional vacuum on the hole shot. Sealing up the pump area better helps. GP1800s were particularly vulnerable to this