Could be egr valve,
Just got this off internet hope it helps
friend's XF had very similar problems...pulling away from junctions or roundabouts and the engine lost power briefly, before springing back to life. The Jag dealership had the car in several times but were unable to find the problem initially, it wasn't untill they had the car in for a few weeks that eventually the cause was discovered. They replaced the EGR valve (Exhaust Gas Recirculation Valve) and everything was fine thereafter. I hope this gives you something to look into.
Or it could be fuel related