Have had irrational fear of certain public speaking all my life sometimes no problem and other times extream to the point of avoidance why?

The why...: Can be discussed with a psychologist which itself can be therapeutic. But at 53 for this degree of stage freight you consider relieving yourself with by taking an as needed beta-blocker (inderal) or other safe medications. This may also be irrational shyness which is a modifiable human trait.

Social Phobia: Many people have anxiety around public speaking and other performance or social activities. Stress reduction techniques such as breathing exercises, meditation, counseling, exercise, good sleep can help. Still some people are troubled by this problem. One way to treat it is to take a medication called Propranolol prior to the speaking engagement, it is a beta blocker and can be quite effective.

Public Speaking: As you probably know public speaking is one of the most feared experiences for most people. Interesting that you are ok sometimes and not others. Try toast masters as it is effective for many people, also there are a number of groups on linkedin that help with advice about public speaking. Your lifestyle is related also, daily exercise, proper sleep make a difference to counteract stress hormones.

CBT: Seek a cognitive-behavioral therapist who specializes in phobias. One method they use is systematic desensitization which slowly and incrementally brings the person closer and closer to the thing they are afraid of in slow, graduated steps that help make the anxiety more tolerable.