CC Display

I just got off the phone with DTV re closed captioning issues. (Had to reset receiver to turn cc back on). On the options for display it gives the choice of background box, I chose transparent (that is my preference, works great on my tv with ota hd channels. ) It works only occasionally, most of the time it displays a solid black box behind the lettering. I asked the guy why this is happening (sometimes switches displays in the middle of the same show) He said the new software upgrade was causing this. I don't believe him. Anybody have experience with this option, any info would be greatly appreciated.