autovac hitting too many tables at once

Hi,

I've got an 8.4.11 system that I am relatively new to and I am seeing multiple autovac processes kick off on several of
thelargest tables at once and it is causing pain. 

Are there any suggestions to

  a) quickly relieve the immediate pain

and

  b) prevent such issues in the future (other than going to manual vacuuming on a schedule).

Thanks,

Greg Williamson