Hi @benoitm
Given that it works OK when in the 1 hour wash cycle mode then it could be a control board problem.
Perhaps inspect the board for any corrosion problems and also that the cable harness connections are also clean and corrosion free.
Here's the [document|32641|tech sheet] for your model. On p.2→p.3 it describes how to remove the control assembly.
On p.6 there's a strip circuit diagram of the wash motor operate circuit which makes it easier to inspect the wiring connections etc for the motor circuit. Although given that the 1 hour wash cycle works OK (every time?) it seems to be OK
If you find a faulty part, here's a link to a [https://www.searspartsdirect.com/model/5ed4b0o5vp-001198/whirlpool-wdt750sahv0-dishwasher-parts|parts supplier] that is ''useful.'' Locate the wanted part in the diagrams, to find the ''manufacturer's part number'' for the part listed with it. Search online for the ''manufacturer's part number only'' to find suppliers that suit you best.