1994 827 sterling going about 50mph the speedo will drop to zero for a few seconds then go back to reading normal no gear change occurs but s4 light flashes on dashboard

i can be driving down the road about 50mph and the speedo needle drops to zero for a few seconds then returns to normal but a green s4 light flashes on the dashboard but it seems to drive ok with no gear change when i stop and turn off the ignition then back on there is no light flashing and it drives normaly