Hi @chrisstillman
Here's the [document|41452|tech sheet] for the dishwasher.
Go to p.3 to view the procedure to run the Diagnostic cycle and check what error codes are shown.
If there's a 4:3 wash motor error code shown go to p.8 to see what to check.
Given that it's seems intermittent, on p.4 there a diagram of the wash motor operate circuit that may also help. Also check for fractured wires in the wiring harness where it leaves the door to enter the body of the machine to get to the wash motor etc and for loose/corroded connections to the control board.
If you find the problem and it's a faulty part that needs replacing, here's the [https://www.searspartsdirect.com/model/4schdkzf42-001198/whirlpool-wdf540padm1-dishwasher-parts|parts list] that is useful. Locate the wanted part in the list and note the part number that is shown with it. Search online for the ''part number only,'' to find suppliers that suit you best.