Cooperative Learning Groups

I currently co-teach a Biology class in which we use cooperative learning (lab) groups at least once per week.  We have various methods of grouping students (groups are typically 3-4 students):

-heterogeneously grouped lab groups (teacher chosen)

-randomly chosen groups:

• students pick names from a jar
•  students count off 1-4
• students match puzzle pieces 

 -homogeneous/heterogeneous groups/partners chosen by students (we find that typically students get a lot less accomplished when they choose their own groups or partner!)

I feel that, when grouped heterogeneously, students tend to be more interested and invested in learning.  Each member of the group is accountable independently as well as on a group level.  We don;t typically assign roles to each student in the group but they kind of take on that role themselves.  The downside to cooperative learning groups is the noise and those few students that constantly need to be prompted back to task.  How do you deal with this?