OK, assuming it's an auto from your comments...
This being the case, your 'box is either in need of a service, or on the way out (and maybe a little overheated at some stage, too).
Time to flick that EGR valve. Not that I said so, but GET RID OF IT!!!

Get a blanking plate to sandwich under where it bolts to the inlet manifold, and fit it using some silicone gasket stuff.
At 150 000, it's time for a new timing chain at least. Running hot can sap power, too. Does it tend to get a little warm (yeah, I know, it's been stinking hot here...)?
Buy brand-name premium petrol. Trust me on this one - also run a couple of bottles of Spitfire or similar through it ASAP.
There's plenty more, but this is a start.
A note on V8s: Clevos are getting long in the tooth, and are way overrated for light duty domestic use. Look up one of the importers and get a near new EFI 302W/AOD, and either run Wolf 3D or go to an emissions carb. Either way, fitting a V8 to an XF is trouble in ACT or NSW, as they never came factory, so compliance/engineering is where the grief lies.
Cheers, Adam.