For a couple of months now, I've had an intermittent problem with black smoke from the exhaust coupled with a reduction in power. Recently it's become less intermittent. Motorway driving is fine as even with the problem happening, I can gradually get the car up to 60mph, put it in cruise control and gently get home. However come off the motorway and try to get up and down steeper hills and it really struggles. My local garage (here in France, and not a Hyundai dealer), did a diagnostic test and identified a "problem" with the Air Mass Sensor - so we replaced it. All was well for a couple of weeks then the problem came back. Garage said new diagnostics test showed "No Faults". They suggested perhaps a problem with dirt in the fuel (diesel, by the way). This weekend, I changed the air filter and the fuel filter under the bonnet and, although I did a 100 mile round trip on the motorway yesterday with no problem, today the problem has reared its ugly head once again. Unfortunately my local garage are closed for hols at the moment so I'd be grateful for any thoughts.