I had pretty much exactly the same problem for a long time, and mine was a simple fix.
Pop your bonnet and have a look between your battery and engine block. On the top of the gear box is what looks like the top of a walking stick, but in metal. Not sure what its called, but beneath the thick end, about 4 inches below it, is a solitary nut. check if its tightened. If its loose, then that will be your problem. Make sure its in the middle of the gap too.
I hope thats whats wrong, as all you need is a spanner!