I would suggest to keep a table with the committees each volunteers belong and the weekly time committed to each committee. Then establish some limitations:
- To encourage diversity in participation establish a maximum to the number of committees a volunteer can belong.
- To guarantee each volunteer can focus on the job required in each committee fix a maximum to the accumulated number of hours he or she can commit to all committees.
I also would suggest to limit the number of members of a committee that can be appointed by the members of the same committee to less than 1/2. The majority of the members should always be appointed by an external body.