Not sure on the XKR but fairly certain on the S Type that one ABS sensor also doubles as the speed sensor. If that's the case, there is an intermittent fault somewhere on that sensor or that circuit. I might be wrong on this so double check your wiring diagram but it seems too much of a coincidence that both the speedo and ABS throw a fault at the same time. Also the PCM usually gets a speed signal from somewhere to tell it about the road speed/engine load etc to determine what gear it should be in so again, this points to one common component - whether that's an ABS sensor or what, i can't say but i know someone had a very similar fault with an S Type not long back and that was (i believe) an ABS sensor OSR if memory serves but might be wrong, been asleep since then!