For whatever reason, the broadcasts at 2100 and 2200 routinely boom in with stronger, cleaner signals than any of the other daily transmissions, regardless of the solar weather and the state of the ionosphere. I tend to find the days where those come in at 16180 and 17480 kHz (Tu, Th, Sa) are maybe a bit better than the other days (11635 and 10715 kHz). If you like decoding the RDFT data bursts, those are the best times to get them. Could they be using different transmitters beamed in different directions?