I'll confess, I recapped this set, it used to be mine.
Kirk, how long does it have to run before the trouble starts?
I fired the set up a few days before I took it to your parents house and it worked OK, I didn't run it a long time though, maybe 15 minutes or so.
I Know it hasn't been run more than an hour total since it was recapped.
I can't imagine what would cause an arc inside the tube other than a bad tube or way too high HV?
There are some adjustments down on the side of the HV cage accessble through the bottom door, check and see if there is a HV adjustment?