I have returned after a couple of years due to Corona lockdown and the game is great again but unplayable. I hold the RMB to move and it acts as though the RMB is intermittently unpressed so I stop moving and randomly select mobs and allies. I cannot play bgs or dungeons as it is worse the game is really unplayable. I have searched forums and tried every fix I can but still having problems. I am going to have to quit unless fixable as this is no good. Help!
Are you sure it’s not just your mouse? Have you tried a different mouse? Have you tried the RMB outside of a game, such as the desktop (just try to constantly select stuff and slowly release pressure off the button to see if it stops working before you fully release the button). It’s possible the switch inside the mouse is defect.
Are you using Razer Naga series? If yes, they are faulty. If not try changing USB ports, possibly with USB 2.0 port.
It happened to me when my mouse became faulty.
I had Corsair Scimitar Pro and my RMB began to act up, by just randomly ‘unclicking’ even though I had it pressed. Same issue as you described above.
It’s more than possible that RMB is just stopping to work as intended. 
I have Logitech Proteus and it works like a charm now.