I had the VUK problem you describe recently myself. I had switched the original Tardis over to a USB Tardis that looks nicer. However, after getting the same VUK problem. I started looking at probably replacing the Optos. However, what I found was that the green and white wire that goes to the blue bulb in the top of the Tardis was interupting (in between) the optos sometimes. With it Ty-wrapped away, the problem went away.