Yep this happend on my XF and now also happens on my AU XR6. After I come off the freeway and then sit in traffic sometimes it happens - it's more prevelant when the air conditioning is switched on, on hot days.
I don't know why this happens - would be interested though!
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.