Andy, I would check for sure that the motors are wired in series before using that decoder. If the motors are in parallel that could allow too much current draw and cause the shutdown. As well make sure the motors turn freely by hand and the grease n the drivetrains hasn't caked up.
I have some old D408SR and Soundtraxx DSX decoders that I've pulled out when I did coreless motor upgrades. But I only have EMD and Alco sounds. I could dig them out and send you a pair if of any interest.
Pete