Citroen C4 tick over and running problems

brought my car 7 months ago second hand. For the first 5 months it ran fine and was a joy to drive but then started to rev itself whilst on tickover. Took it to a garage who saw it revving up and down on its own but when they plugged in their computer it came up with no faults. Now when I drive it jerks along and I feel as though I'm fighting to keep it going but the garage say that until it diagnoses itself with a fault then it will just cost me to have parts replaced needlessly as they don't know what is causing it to run so badly. They say the engine is doing something called 'hunting' but don't know why. Any suggestions as I'm getting stressed out whenever I drive my car!