1. For you isolationists, saloon and gearbox are normal English car words. They just happen to be used in, well, England.
2. The shift points are controlled by the control pressure cable running from the throttle linkage to the transmission. This is the first place to check and adjust it to the correct freeplay. Since you said the car initially shifted OK, I am doubtful that this will be the cause, but make sure it works OK, is connected at both ends, and is set to the correct freeplay.
3. If it's still "bunking" (English, 2-year-old for "broken"), then you need to find a good transmission specialist.

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Prost!