I have exactly the same symptoms!
Yep, I've just replaced the sender unit - $40 (pain in the arse job) and it has made no difference.
Can anyone tell me what's involved in doing "better earthing" ?
Wheels: 17" Series 1 XR
Exhaust: VCT with no resonators & 3" mandrel from mufflers back, dump pipe
Pacemaker headers (Waste of money).
Air Intake: T-Series, K&N Filter.
Brakes: Calipers painted red, DBA slotted rotors & Lucas pads.
Fuel: Dual Fuel.
Dyno: 143 rwKw (Dynodynamics - shootout mode).
Heathcote: 15.87 @ 89.1 Mph.