Alternating while booting up is normal, however, during regular operations, it is not normal for it to flash back and forth. There isn't a way to adjust this, and alternating in regular operation is a sign that the unit needs to be serviced.
I might suggest filling out a service transaction for your product, or contacting service. Unfortunately, I don't know of any other way to fix the issue:
Samsung Model 9308. Recently, during normal operation, my monitor continues to flash between Analog and Digital mode. A box appears in the upper left hand corner of the screen and alternates between analog and digital. This happens both during start-up and normal operation and continues sporadically throughout use. How do I correct this?