Control of Denon 4308 and 3808

I had no problem setting them up. In fact - one of these was the first thing I set up prior to deciding to purchase the upgrades. Now I've found that the control over both of these receivers has been 'lost' for lack of a better term. I one of the receives by unplugging it for 30 seconds and I was able to re-establish control. But a day later it was last again. What's happening?

In general, the further you go back, the less reliable you may find the Denon models. This can also be said of Pioneer of course and some others.

In general, the correct procedure is to unplug the receiver if it stops responding. Make sure you have fully up to date firmware on the receiver as many such issues have been fixed by the manufacturers over time. One option to consider is serial control as most of the connectivity issues were related to these manufacturers early attempts at IP control which means serial control bypasses those receiver bugs.