OK, gotta say that mine is between 66% and 72%consistantly for last few years with no problems---BUT . . .
I have found that almost any DVR that has very little space left (say at 85-90's% for some time) and even at my lower 66-72% over a LONGER time is going to stress the HDD as far as a lack of wear-leveling will occur to the point of the HDD crashing, and I am seeing what I believe to be the effects of no proper wear-leveling on the same portion of the HDD I keep on hammeing away for "disposable" recordings whilst I am keeping the long ago recordings on the HDD that has not seen new action in years. I have experienced this MANY times over the years as a Dish DVR subscriber.
OP, the fact that the problems seem to clear up after creating more HDD space by transferring recordings does seem to suggest that the portion of the HDD where the DVR places new recordings is likely to fail in the not too distant future and I would suspect for not being able to apply proper wear-leveling.
This is the bane of DVR's. We all either want to keep recordings for repeat viewing or we keep them for a very long time (as in YEARS because it takes me that long sometimes to get the time to watch that movie or TV series), yet stuffing that HDD and leaving about a third--or LESS--of the HDD for repeated new recordings and deletions means we are battering the heck out of a small portion of a pretty big HDD and we lose the benifit of wear-leveling. I've learned my lesson to never let the HDD get up to the 80's or 90's %, and while my self imposed limit of 70% filled HDD has gotten me further along than in my earlier 90% filled Dish DVR HDD years, I can see all the signs of HDD problems rearing their heads, but when I did dump a fair amount of content that I had finally watched or decided the shows really weren't worth waiting to see, I also noticed improved HDD performance--fresh HDD territory to use now for wear-leveling, but filling it back up to 66-72% has brought back the minor problems.
And let's remember that the DVR is also writing to the HDD even when we watch Live TV each day for hours at at time, so relatively small portion left on the HDD for writing data gets beaten up just watching Live TV and the DVR attempts wear-leveling there as well, but can only do so much with the restricted space left for writing. Curses, all these HDD's fail at some time.