I have a Modify S110+ in a V3 gearbox shooting at 407fps.
I think you could have introduced a leak while installing the spring.
A few things to check..
- Piston o-ring. Make sure it makes an awesome seal. If not, stretch it with fire or replace it with one of those nice MASK ones Jugglez sells on airsoftparts.ca
- Make sure your hopup is pressed against your gearbox when the gun is assembled. I'm not familiar with P90 / version 6 hopups, but if there's any way you can increase the tension of your hopup against your gearbox so that the nozzle makes a tighter seal against the bucking lips, do it.
- basic sanity checks: turn off hopup while chronoing, use 0.20g BBs
