Raising the pump lowers water pressure (in this case weight of water) around it. Cavitation appears when pressure (usually at suction side) drops so much water vaporises. So it sounds clear enough your pump is cavitating, and lifting it just made that worse.
I guess you can eliminate the cavitation by increasing fluid volume in your system or by... Well, we're both over our heads *ack* so try Google and good luck.
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