Remove the interior gearstick gaitor. Get some silicone spray grease from Halfords (comes in yellow tin with 'rubber and plastic' grease written on it, same stuff as the grease used for window runnders etc.) and spray all over the plastic with in this. As what is happening is the gear lever selector gate hs become dry.
This special grease is long lasting and specially designed for the job. Give it a go.