Bluetooth keeps dropping?

So I've never really used Bluetooth in my 2009 Mac Pro. Performance was spotty when I first got

it (refurbished), so I've always used a wired mouse and keyboard. Anyway, I recently thought I would

try a USB dongle to see if that made a difference.

After the dongle (IOGEAR GBU521), pairing with my iPhone and iPad was fast and easy. But.....it immediately

loses the connection after pairing. And I usually get the message "Network is unavailable".

I've tried the dongle is various places: on the front of the computer, on a USB hub, on the keyboard, etc. I

even used Bluetooth Explorer to confirm that the IOGEAR is the active Bluetooth controller.

Any ideas, besides installing a new WiFi/BT card like the one (currently out of stock) from macvidcards.com?
Is it worth trying another dongle? Maybe disconnecting the internal BT card?

BTW, I've tried turning Bluetooth off and on, deleting and re-pairing the devices, resetting the SMC and the PRAM.

Also, deleted the com.apple.Bluetooth.plist file. I'm running El Cap 10.11.1.