Sometimes when I put my foot down in my S1 it reads 9 psi on the gauge but with what sounds like air escaping and it accelerates ok, then all of a sudden its like something snaps shut and its off like a rocket. Any ideas and help would be gratefully received.

 

The usual checks, split/perished hoses, air leaks, and it could have something to do with the actuator or Amal Valve (if it still has it)