Losing particular gears can be a symptom of internal transmission damage, but it can also indicate a simpler problem with the selector mechanism that joins the gear stick to the gearbox. However, if this was the case, you’d probably lose two gears on the same selector plane (such as first and second or third and fourth) not first and fifth which are at opposite ends of the shifter mechanism.

If there was internal gearbox damage, you’d possibly also be noting other noises and problems caused by the internal carnage, which makes me think the internal selector mechanism might be at fault. At which point, it’s probably a gearbox-out repair.

However, a transmission specialist that knows this gearbox well may have a different opinion and should be your first port of call. A specialist will save you time and money in the long run.

A build up of carbon deposits (a by-product of burning petrol) is, indeed, a known fault with this engine. If it occurs, it can create the precise symptom you’re seeing (the check-engine light) but can also lead to an erratic idle and misfiring. There have also been cases where this engine has developed wear in the exhaust valve and valve-seat region, with similar end results. Unfortunately, the only fix is to remove the cylinder head and replace both the exhaust valves and their seats.

I agree that 52,000km is a depressingly low mileage for problems like these to occur, so you need to make absolutely sure it really is the cause before you start to tear into the engine. A leak-down and compression test may throw some light on things, as can an electronic scan of the car. A fault-code of P0300 is a big clue that the exhaust valves are at the centre of the problem. This model Barina was another Holden that was simply a re-badged South Korean-built Daewoo with all that implies.

A car that feels unstable even in a straight line is a potential death trap. The higher speeds on the open road (as opposed to driving around the suburbs) are obviously amplifying the problem, but whatever it is needs to be fixed fast. To be honest, I wouldn’t be driving it at all.

You could be looking at a problem with either the steering system itself, worn suspension or even a loose (or rusted) front cross-member that is allowing things to move in directions they shouldn’t. Don’t rule out poor wheel alignment or even incorrect tyre pressures, either. Has the car been crashed and repaired? Poor crash repairs can leave a car misaligned, making it steer and handle oddly. Either way, get it checked and fixed pronto.