I was having similar issues, using LMB +RMB to run/kite if i let go of one or the other it would act like my mouse was accelerating @ 0 acceleration and 600 DPI. with ingame mouse sensitivity turned off.
POSSIBLE FIX.
I made a wow profile specific (using razer synapse) most mouse software have something of the sort to apply settings when a specific game is open.
did some simple testing and was having this issues at polling rates of 1000 and 500
BUT NOT 125. the problems went away at 125. it seems less polling = less signals sent = less mouse spazzing. not sure if its a for sure thing but its working for me.