Over-heating and coolant blow-out through overflow on 1994 GMC Vandura 3500

Our van will overheat frequently and VOMIT out anti-freeze through the overflow. Very annoying. I don't think it's the head gasket, as it doesn't do it constantly.... Thermostat? Water pump? Something else?

Asked by for the 1994 GMC Vandura 3500
head gaskets