Shower pressure problem? The shower runs from the main hot and cold marine system....

The shower runs from the main hot and cold marine system. The pressure is normally fine but every couple of weeks it loses deeply of pressure. When the head is lowered the pressure raise, when its raised the pressure drops again. I am hugely tall and necessitate the shower to be as high as possible. Anybody know a possible remedy for my problem ?
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Answers:    I enjoy never had one do that when you alter the position of the head but I would bear it off and pinch it apart and see if there is any iron, trash or otherwise contained by the head that could be cause it.
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