Mini Box Troubles

When I installed our Mini Boxes we received all channels with no problems. Since then, one by one, several channels began tiling and then the signal just stopped - became "unavailable". I went through all the proposed solutions from home, and then Cox had two different technicians come out on two different days. Problem is still not solved. Fronty seemed to have a similar problem that was resolved as documented in this forum on April 25, 2015. I wonder if the issue he solved involved an HD TV, and what kind of high-tech coax cable he used to solve his problem. Mine is an HDTV(actually 2 of them) and my mini boxes are connected directly toward the wall. I am using 6' Quad Shield RG6 coax cables on them. We have another non-HD TV in the home that receives the signal through a regular full sized box from Cox and we have very few problems receiving signals on that set.