The accelerator of my Peugeot is sticking on long drives?

The accelerator of my Peugeot 106 1.5D keeps sticking when I drive over long distances. When I try to slow down or engage the clutch, the engine is still racing at high revs. I have to put my foot under the pedal to pull it up in order to decelerate. I have lubricated the various parts of the accelerator with WD40 but with little success. Could anyone offer some advice? Thank you.