A very common cause of problems is that your external USB Bluetooth adapter is located very close to an active USB 3 port. If this is the case, even if the USB 3 port is only used sometimes its going to be best to move your Bluetooth adapter to another USB socket as far away as possible.
On a desktop computer
Try moving the location of any plugged in USB 3 devices on your desktop computer. In this example they are being removed from the rear of the PC
If the two devices are plugged in next to each other try separating them as much as possible by using USB ports on different locations of the desktop.
Many desktop computers have USB sockets on the front panel and around the back where most of the other sockets are located. With this in mind try placing your USB adapter on the front of the computer (as it's most likely going to be a little closer to various Bluetooth devices you have on your desk) and plug the USB 3.0 device in to a compatible socket on the back.
If this exact configuration doesn't work try switching them around. Remember, overall distance between the two items is the key here so the bigger the space between the USB Bluetooth adapter and the USB 3.0 port and device the better.
On a laptop
Here a USB Bluetooth adapter can be seen plugged in on the right hand side of the laptop near the Bluetooth mouse it communicates with. A USB 3 external hard drive has been plugged in on the left hand side to separate the USB and Bluetooth as much as possible
Normally all your USB ports are going to be fairly close together and you may only have one or two on a small laptop to choose from.
Still it's worth trying moving the USB Bluetooth Adapter around into other free USB ports just to see if the difference in distance between components helps to mitigate the problem.