If it happens only 4-6 months, is there anything that changes in your diet, environment, stress levels, etc? Also if you are a woman is there any other symptoms like hormonal change? Why would parasites bother you only once or twice a year. These buggers grow and propagate once they are in your body. Also keeping a diary of what you eat may help you if foods trigger this reaction.
Doing a cleanse will certainly help. Also check the Liver Flush forum.