This sounds very much like a fueling problem, if the car has a carb I would suggest that it has a partially blocked jet, this would tally with the description of it being ok when you are cruising. If this is so, unless you know what you are doing I would let a garage sort it out for you, it should not be a very expensive job, they will remove the jet from the carb and blow it clear with some comporessed air. If the car has fuel injection, try a bottle of Red-X injector cleaner from Halfords, you just poor it into the petrol tank with the petrol.