Mark,
You may want to try changing the clock frequency for the SPI interface. I solved my problem by merely changing the clock frequency from 1000 KHz to 1250 KHz. I found It worked at 4000 KHz down to about 1150 KHz. E.g.:
SPI_CS_CONFIG = new SPI.Configuration(MP3_XCS_PIN, false, 0, 0, false, true, 1000, SPI.SPI_module.SPI1); // fails 9 of 10 trials
SPI_CS_CONFIG = new SPI.Configuration(MP3_XCS_PIN, false, 0, 0, false, true, 1250, SPI.SPI_module.SPI1); // works 10/10 trials
Hope this helps as a workaround,
Quentin