I had the exact same problem. It is caused by electrical interference somewhere along the hearbeat cable. Mine also happened only at night and only in winter.
Turns out it was one section of our outside lights. As soon as the lights timer went on, the comm problems started when tv and HT was on. Switching the primary and secondary decoder didn't help.
To make it more bizarre, it only happened when the outside lights and my home theatre was switched on!! TV only. No issues. Removed that section of lights, boom no problem. I struggled for months and stumbled on the solution by accident. Eventually started to notice a pattern that the problem was gone when manually switching lights of for unrelated reasons every now and then.
Removed those lights about a year ago. Never had problems since.
Not sure why that combination and only in winter but I suspect there is a sensitive interference point and cold nights affecting the performance of the outside lights together with a frequency of just that amount of electricity caused problems. But thats just my own conspiracy theories.