Questions: Do they have neighbors? How far away? Does it happen around certain times of the day? Where is the dish mounted? What is near the dish?
My 1st thought goes back to a couple of installs many years ago. When a radar detector sitting on the dash of a car, was causing the signal on the dish to screw up. In one case, it was the neighbors car - about 60 feet away from the dish. In another case it was the owner's car, parked about 80-90 feet from the dish. I wouldn't think any of those 'old' radar detectors are still around - there was regulation decades ago, that said 'new' radar detectors had to be built with no detectable emissions. Which wasn't that hard as many companies had already started building them, due to states with laws against them using radar detector detectors. The point being, good possibility it is a rouge electronic device causing the issue. Remember the thread a year or 2 ago about the garage door opener, that was causing a CCTV camera location not to work. Crazy.
