Garmin 305 Weird Speed/Pace Issues

[Deleted User]

Not sure if anyone can help with this ?? I use my Garmin 305 for running and cycling and went down a huge pothole that I didnt see on  my Road Bike earlier today.

Since then I have had very erratic speed/pace reading - difference in 1/2 mph over a distance of about 100m and even with the device just left on a table I have seen speeds of up to 4mph !!. Have tried a hard reset to no avail