might want to check the VRM temps. VRM 1 in particular. if the cores getting hot I'm sure they will be very very hot. 125C kinda hot
O and my 290x has a similar problem as yours get some black screens while my 4k monitor reconnects and some amd message about errors when waking it from sleep mode. usually just reboot and everything runs fine and it's never done it while gaming. think it's the card not getting enough juice and some energy saving features messing with it.
or hell maybe it's just AMD drivers
Whats the best program to check vrm temps on? ive just been using after burner