Also, can you remember the following: when I put my thumb over the nozzle on the mount itself (i e the nozzle that the vac pipe would go onto), I feel fluttering air coming out of the mount, this suggests that while idling the spring in the mount is bouncing up and down a little. Hastens in both mounts.

Did you ever check if your mounts did this?

If this bouncing of the spring is basically allowing fluid to move into and out of the chamber that controls soft/hard setting, then surely this is a problem on idle? Would this fluttering not occur if the vacuum system after the mounts is air tight?