Tbirdman,
Most likely causes, combustion gasses getting into the coolant via a head gasket or block / head leak or air getting pulled in though the pump packing.
I'd check for combustion gas leaks first using a simple tester from NAPA:
http://www.arrowheadradiator.com/head_gasket_or_combustion_leak_test.htm Also I'd make sure the fan belt is tight, 1/2" to 5/8" play when pushed on, on the right side. If the belt is loose it may not be circulating enough coolant and pulling enough air thru the radiator core at lower engine speeds, causing coolant to boil in the back of the block and show up as foam in the top of the radiator.
Regards,
Dave
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