Depends what gear you were in at the time. I have had this kind of thing. I have a 2.0 TDCI and if I hit resume on the cruise control (when in say 6th at 60 and with a heavy load), the car seems to stop accelerating back up the predetermined speed. Perhaps its a protection thing. I know mine is with the cruise control on and your situation may be without cruise on. With yours being a 1.8 and having 6 people up, it may be this, or just a slight flat spot. I also have a Mk3 Mondeo and have on occasion had a flat spot at around 60 or so.