could be the mount went bad, never heard of shimming a mount before. maybe its time you did the work and then you will know whats going on, motor mounts like a 30 minute job first time out, basic tools are needed. and its easy to change really. i did both motor and trans mounts in less then a hr my first time out.
you may also want to check for oil leaks, as a oil leak from the v/c can leak to the mount and the oil on it will break down the rubber in the mount and kill it.
then flip the dog bone bushing on the rad mount, that firms things up nice too.
