'99 ford fiesta revs accelerate when in neutral

my fiesta has just developed a poblem that during long motorway journeys when I go into neutral the revs increase to varing figures then die away.
when I push the clutch in the rev also seem to die but settle slightly higher than idle. could this be just a sticky throttle cable or something more sinister
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.