Intermittent cruise control

2004 530i. My standard cruise control is behaving oddly. First, it when it was set and I activated a turn signal it would turn off but could be reset. Next, it turned off by itself and could not be reset. Next time car was used, worked normally.

First, what is this? Second, how do I test to see if it is the switch or the module? And, last, where is the best place get discount parts? Thanks in advance for your help.