No power to windows, AM/FM Radio, or mirrors.

That is an odd problem. I had the same issues once, never even noticed if the radio was not working but my windows and mirrors had stopped working. It just turned out to be a bad driver's side window/mirror control. I tried the passenger side window, it worked. Unhooked that control and plugged it in on the driver's side and it worked for the window. I ordered a new driver's side control and all was well. The radio not working at the same time is a head scratcher though. There are some relays in the same place as the fuse box. On top of your dash there should be a little built in tray with a pen holder, or on some models that is where the CB radio mount is. Pull out the cup holder right there and there will be two torx bit screws, loosen them and pull the cover off. Fuses and some relays right there. Hope any of that helps somewhat.

 

might check circit breaker fuse for acc's

 

The Volvo has a fuse block located under the hood driver side and one in the storage box drivers side. The radio part is weird but your mirrors and windows are all controlled through the drivers side door switch I believe. If this is bad it could cause the issue you are explaining. I have not heard of it being intermediate normally it works or not.

 

Believe i or not, I would check your batteries. The accessory relay kicks out when voltage gets low (somewhere around 12 v) The relay sounds like it is working fine and you are running on low voltage. If the batteries are not holding a charge I think it would cause this problem. Just my humble quess

 

Oh, and as stated above by rogueX, the relay is in the middle of your dash behind the cup holder. It is the big yellow block on the left side of the fuse panel

 

I was also thinking a relay, There is a schematic on the cover, also try when this happens if opening or banging on the door panel will do any good, this would say a connection in the door? I'm not sure about the radio except that it was "T'd" into the power line of the door assembly but I'm not sure, I hope you find it, Glad it isn't your Jakes that are intermittent!