yea, had that problem when my speedo sensor was playing up.
Started out occasionally loosing speedo for a split second (sending gearbox into limp home mode) then eventually lost speedo all together! Needed a new speedo transducer.
EB1 3.9L MPI Falcon Wagon with converted T5 / Pacemaker & 2.5" Redback sports exhaust / 2" lowered blocks at the rear, Kings lows up front / ED Tickford Head and Cam / K&N panel filter with XH intake Snorkel / Full EL Fairmont interior with woodgrain
89 EA1 3.9L TBI Falcon Sedan with 3 speed auto / currently being upgraded...