Guys I have a 2008 Isuzu Sportivo AT. The following was recently done to my ride:
1. Changed Oil.
2. Change fuel and oil filters.
3. Cleaned air filter.
4. Blanked EGR (This removed the black smoke).

On our recent trip to Naga City via south road I experienced the following:
1. Maingay na makita.
2. Very slow acceleration (Feels like stuck to the 1st gear).
3. High fuel consumption.
4. At 80-90kph it feels like from the 4th gear it automatically gears down to 3rd while accelerating.

On our way back eto naman ang nangyari:
1. Pumino tunog ng makina hindi na maingay.
2. Overtaking power was restored.
3. Fuel consumption improved. We managed to consume a little less than 1 full tank of diesel.

Question: Is it possible that a broken part was suddenly fixed? No this is not possible.

Is so ano kaya ito mga sir? I don't know. Car needs to be checked.

My concern is the downshifting na alanganin ng AT on high speeds. Is this the injection pump or the AR transmission? It could be that the throttle potentiometer needs adjustment or replacement if your car has this feature, or the transmission shift cable need adjustment.

I forgot to change the AT fluids pala.