I believe you may have an air leak somewhere in your system before the pump impeller. I experienced the same situation as you last year, it ended up being the pipe that comes into my superpump strainer basket I found a crack and it was letting a little air in which gets trapped there eventually forcing the pump to lose prime. If you can post a reply with your equipment the board may be able to help you better as some equipment have known trouble items. Good luck.
Regards,
Mark
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