8xx spi completion sometime doesn't generate an interrupt

As far I understood the spi bus device of mpc8xx works independently from the 
attached device (in my case a dataflash m25pe80). Thus I don't understand why 
sometime when I start an SPI transfer I don't receive an interrupt for the 
completion of this operation. I wait 50 ms for this interrupt and sometime it 
doesn't happen. Anyone had a similar problem?

Bye,
Antonio.