Simon,
The values you are receiving indicate that you are most likely mixing units of x & y with z. I would guess your DEM is in a geographic coordinate system using units of decimal degrees, and your z (pixel) values are in meters/feet. To ensure accurate slope values you need to have x,y, and z all be in the same units. So, what you need to do is run Project Raster and specify a coordinate system that has the same linear unit as your z units in the DEM.
Regards,
Eric