My gearbox (series 3 petrol) suddenly decided not to go into 3rd or 4th today, 1st 2nd and reverse are fine still. I have stripped the top cover of the box and checked the detent springs and balls, they seem ok. I have read that the problem may be the synchro springs broken or jammed on the 3rd/4th Synchroniser. Has anyone else experienced this problem? If you think it may be the synchro clips do you have any tips for changing them and do you know where I can purchase some?