1997 Citroen Saxo 1.4 VSX 44,000 miles?

I have an engine misfire on my R reg Citroen Saxo when accelerating. when I pull away say from traffic lights the car will accelerate quite quickly with no sign of a misfire but when I cruise at between 50 & 60 mph and then need to accelerate that is when the misfire comes in then if I ease my foot off the throttle pedal slightly the misfire stops. other than this the car performs quite well.