Problems with the speedometer, firstly it was not working?

Hi I have a Corsa club 16v, 1199cc, first registered in early 2000, I have been having problems with the speedometer, firstly it was not working I took it to a garage and they changed the fuse that worked for a few weeks but every now and then it would go a bit crazy and bounce around but correct itself after a few seconds, then after a week or 2 every time it would bounce around the engine management light would flash with it and the car would act as though it was loosing power and would almost bunny hop up the road, I took it to the garage again and they said it was a common fault in corsas, they upped the revs and that seemed to work too until now, now when I start the car it works fine for about 5-10 mins but then drops to zero and stays there and the steering on the car seems to go heavy, nobody seems to know what's wrong. Can you help with this is it something I could fix my brother inlaw is newly qualified and could help if he new how.