Not necessarily cavitation but something similar. Could you describe what the plumbing looks like from the sump to the pump? Likely this part of the plumbing has too much restriction which results in a low pressure situation. If the pressure is low enough dissolved oxygen will actually be liberated from the water, a very common problem that most never figure out and usually blame it on a pin hole which is actually pretty ridiculous when you think about it.
A quick fix is restricting the output flow of the pump as already mentioned, this will in turn increased the pump intake pressure. A better fix is to plumb the pump properly.
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