creeative

Hi there When I first bought her 18 months ago, on the odd occassion (usually motoring up the M5) she would drop out of 5th gear. Then 5th gear became very difficult to find and now it's gone completely! All the gears are feeling a bit stiff now and she crunches into reverse gear. I just wanted a bit of info before I take her into the garage. Does anyone think this is reparable or does it mean a whole new gearbox? Any ideas on cost? Thanks in advance.