I recommend a bypass on every dimmer.
So for a test, turn off the breaker for the zen77, count to ten, and then turn it back on.
Does this clear up the behaviour for a day or so?
I only have one zen77 but have never had an issue with it. My understanding is that the 77 and 76 are identical to the 27 and 26 electrically with just the zwave upgraded from the 500 to the 700 chip. I have three 27s and around twenty of the 26s.
If these switches do not have sufficient loads on them to allow a few milliamps of current while in the off state, it is my experience that the uC gets more and more out of sorts over time.
Another way to verify if this issue is affecting you is to replace the bulb temporarily with an incandescent bulb and see if the behaviour persists (again after a power cycle).
By default, I now always install a bypass on these switches.
I have some background info here: