Do you know the current state of Connected Standby setting? (LP0 state in the BIOS)
If that’s off, it could possibly interfere with apps that run all the time over WiFi like Dropbox.
Though, I’m beginning to wonder if it’s a mash-up of settings that don’t work well together, like enabling LP0 while setting WiFi card to (not) “Allow this device to wake the computer” and/or (not) “Allow the computer to turn off this device to save power”.
Do you have some other programs besides Dropbox installed that keep an icon in the system tray? If so, could you list them? I could easily see Dropbox using the WiFi while Windows is trying to Sleep, sending the WiFi card in to a funky state.