A cold snap is coming for many parts of the country.

Cavitation will not happen in very cold weather so in low temperatures do a test safely climbing hills on lesser travelled roads.
Sudden shut down should not happen when very cold. Maybe add weight to the cargo box to increase engine load.

Then when it is warm redo the test over the same route.

Troubleshoot every other possibility
because it is a lot of work to remove the submerged pump assembly,
and I bet it now costs way more than the $280 I paid.