Actually, the most likely cause is accumulated air in your filter.

You can check this by opening the filter air relief valve while after the pump has been running for a couple of hours. Most likely, you'll be able to bleed air ought for quite awhile. If so, you've found your cause.

However, you have not found the problem, which is air getting into the suction line, via a small leak. Large suction leaks cause you to lose prime, so this leak by definition has to be small. Small suction leaks are a pain to find. Two common locations for a first look are the pump basket seal, and the threaded connection between the pump and the suction piping.