Okay... when it fluctuates, is it jumping up or dropping down? And how much?
up=likely the speedo, down=likely the gear
Stiff cable from firewall to tranny, probably a plastic pickup at end of
cable.. and two wires from it, unbolt that.. it's a nylon or composite
slanted tooth gear.
Also... drive it on freeway, and check odometer against mile markers while it
fluctuates... if odo is right (+/- 5 %) then problem is in the meter.