Ford Puma speedo 'drops out'&sits at zero even at high speed.Will then lose power&stall when I slow down?Help?

Speedo will suddenly 'drop out' and sit at zero, no matter what speed I am driving. There is a slight 'tug' on the engine when this happens. When I slow down, the car loses all power and then stalls. Seems to be no pattern to this happening - just erratic.Engine management check highlighted speedo sensor fault. This has been replaced but problem still occurring. Any ideas??