How can i fix my Ford Fiesta 1995 1.1 LX?

i have the same trouble as above, the tickover speed is too high (about 1500) and it too seems to stick at different revs speeds. When i take it out of gear after driving it, the revs shoot up to 3500 and eventually settle back at 1500. I had this trouble about 6 months ago, and was told to get a new single point injection unit which solved the problem. unfortunately it has returned.