This sounds like it could be one of the known issues with the current 700-series controller firmware (I assume you're using a model C-7 hub; earlier ones have 500-series). People have noticed problems for a while, including Hubitat users as probably the first commercial hub to use these chips, but it seems like Silicon Labs (the Z-Wave authority) is just now getting down to possible causes of the problem.
Here is a recent thread that discussed a firmware update that is reported to fix it:
Here is some discussion on HomeSeer, another vendor that seems to have identified the same issue:
Presumably, Hubitat will test this Z-Wave radio firmware update and release it as part of a future hub platform version if they see any improvements (or at least no problems), but you'd have to wait until then to see.
In the meantime, you have some suggestions above for how to help. In general, the less "chatty" your network is, the better it is likely to behave (this is true in general but seems particularly problematic here). The presence of any "ghosts" or failed pairings has also been suggested to cause problems, so the advice above to remove those should also help.
Another thing you can do is to "tone down" any devices that might be particularly chatty. For example, do you have any power-metering devices? (Some smart plugs and switches/dimmers, including the Red Series products by Inovelli, have this enabled by default.) If so, and you don't have a use for that information, I'd suggest disabling it in the device preferences. If you do need it, try using just the thresholds you need to get data usable for whatever your use case is.
For the hub shutdown, 30 seconds without power (unplugged--after a "polite" shut down from Settings, of course) should be sufficient to reboot the Z-Wave radio when plugged back in; the 20-30 minutes thing is more of a Zigbee "panic mode" recommendation to help devices find new routes (certainly something you can do if you want, but not necessary if your concern is only Z-Wave). This shouldn't be necessary to do on a regular basis, but it can't hurt to try once if you're concerned the radio crashed (you'd see a System Event for this) or if you just wanted to see if it helps anyway.
Otherwise, I'm hopeful the updated firmware will help...some day. 
It's not super-clear that this issue was actually addressed, though some people on other controllers do note an improvement. If not, Silicon Labs seems to finally at least be tackling the problems with this chipset (just in time for the 800 series, ha).
