Hello Docjoe...your unknown issue has stumped many Discus hobbyist over the years and really difficult to pin-point the specific cause.
Many first look at water source and quality. Since you don't age you tap have you noticed if micro bubbles are infested on Discus body which may irritate them and causing the darting. Another thing that comes to mind is possible leaking voltage from a faulty water pump or even heater causing the darting.
Darting fish into tank ornaments or glass pane can potentially kill them.
Another thought is parasites. Raising temps for a period of 3 days or so is worth a try without directly resorting to wormer meds.
Wish I had something more to offer.
...Ralph