I did remove all traces of the old nVidia software and drivers before installing the Radeon drivers and software
Did you use DDU for this, or did you try to do it "manually"?
AFAIK manufacturers don't really bother making the uninstallers of their graphic drivers (as well as many other types of drivers) actually clean up the system completely of everything that the installer and all the myriads of driver updates have done, restoring the system to a completely clean slate as it was before the driver was installed for the first time, and thus they may leave behind lots of registry entries and settings files that might interfere with other graphics drivers (or even updated versions of the same driver, sometimes). It's always safest to use DDU to clean up the system from all traces of graphics drivers, as it has been designed precisely for that purpose and will scour out even the last bit that those drivers leave behind.
As for getting occasional black screens for a few seconds... have you checked it's not a hardware problem? More precisely, your DisplayPort or HDMI cables (whichever you are using)? The bandwidth rating of those cables actually matters, and if you use a cable that does not support the transfer rates you need, that's precisely one of the typical symptoms you get, ie. occasional black screens. (I don't know why switching the GPU would affect this, if you are still using the same display resolution and refresh rates, but who knows. Different GPUs may have components of different quality and may be more susceptible to artifacts caused by poor-quality cables...)
