Have posted this problem last year.The engine seems to over rev erratically,particularly when changing gear.Garage suggested that it was the throttle position sensor and replaced it.seemed to solve the problem but then over the summer the problem seemed to reappear periodically and seemed worse the hotter the weather.Recently I have taken my foot of the accelarator at 1500 RPM in fifth and the revs have remained at that level with no decelaration (surely not right).I have also taken foot off accelarator and dipped clutch and the revs have increased before decreasing.Anyone out there got any suggestions.I noticed that Blue boy has had something similar and it was suggested a torn inlet manifold gasket ?