Hey there,
Depending on circumstances what you are experiencing could be caused by a number of things. Without any details on the system though it would be tough to narrow that down. Typically these types of issues come down to drivers or OS being outdated, Corruption with the game client or settings or interference from other software on the system.
Occasional drops that don’t seem to follow any specific pattern and last a few minutes then go away. Leads me to think there is something on the system affecting performance whenever it does something in the background.
But that is just a guess based on the details provided. I recommend temp disabling any other software running on the system while playing and seeing if that helps. If not then try to get us a dxdiag. posted here or hosted on a site like paste bin.