Ford Focus 1.6 Zetec Automatic (2002 / Petrol)

I have been experiencing this issue with my Focus for the last few months and it's now driving me insane, any help would be greatly appreciated!!

Firstly, the car drives well without any real noticeable issues at all when driving but the speedo needle only seems to increase / decrease in blocks of around 10 MPH so I never really know the true speed that I'm doing! I have noticed though that when putting car into park gear (automatic) the engine power seems to drop considerably for a second and then everything is back to normal, it's almost as if it's about to stall but it never has stalled and I don't experience any power issues when actually driving the car.

I've done a little bit of my own research into the issue and I seem to be drawn towards possible vehicle speed sensor as I've read numerous posts with the same symptoms but a lot also have worse symptoms such as complete power loss, odometer reading loss etc. When requesting a quote for vehicle speed sensor, I received a quote for the following part number located here - http://www.autopartoo.com/oem/ford/4826701.html

Any help or pointers in the right direction would be greatly appreciated!

Cheers,

Craig