I had foam once in my res. Turned out that with my res. setup so that the intake line was placed above the water line and WW in the coolant, the stuff would foam. I ignored it for a day and it jsut got worse. Tried capping the res. off to control the foam and cracked the bottle
Solution:
Moved the intake beneath the water line in the res.
Problem solved.
Make sure that you are not getting any air in the lines or causing enough turbulance to get air mixed into the coolant.