Based on the information you have provided, your father could be having some symptoms of a psychiatric disorder. It could be related to his underlying personality as well. Episodic worsening of symptoms every 15 days that you mention could reflect an episodic kind of psychiatric disorder. An underlying medical condition can also have features of episodic worsening of behavior in certain individuals. Considering that the problem is since 25 years, it may be that the psychological problem could have become more severe over the years and needs to be evaluated in detail by a psychiatrist.
Next Steps
Kindly consult a psychiatrist in your locality in your city who can clinically assess your father and provide him treatment accordingly based on symptoms and clinical evaluation.
Health Tips
Kindly be kind and sympathetic towards your father. Kindly understand that certain behaviors he may be showing could be related to his underlying problems. Try to be supportive. Avoid challenging him when he appears angry as it can escalate the anger more.