Over reving Mazda MX5

[jocass]

I have a G reg Mazda MX5 Eunos. Slight prob, only started in the last 4-6 weeks. After driving fine for 15 min or so the rev's start to increase on their own when stationary at traffic lights / in traffic jams when in neutral.
Ticking over by this time is at 2000 revs & it only does this usually at 30mph in 3rd gear! Is this a minor /major problem? Whats the matter with it an how can I fix it? It's costing me a fortune in petrol! Ta, Jo.