Be careful not to blame the mounts too quickly - The vibration has to come from somewhere.
First of all, make sure the props are balanced. If you are still experiencing vibrations, try rotating the prop in relation to the motor by 30 degrees at the time until best setting.
Last resort is using quality props - Graupner 8x5 E-PROPs are very good, and you can even have them in a counter rotating pair.
When you have achieved a reasonably balanced motor/prop combo, then you can look into the mounts; I have secured the front part of the mount with hot glue, which can be removed if need be. Furthermore, I have added a lower foam bridge between the open, lower nacelle halfs. Also hot-glued in place. Runs very, very smoothly now
EDIT: And do NOT fly with the described vibrations!