Loosing Power when gearing down

I have a 2005 Kia Sportage 2lt AWD, manual gearbox, 230 000 km on the clock.

When i go uphill or try to over take and need to gear back my car seems to loose power.

Its almost as it goes into limp mode. The rpm does not go above about 2500 even with my foot flat on the pedal, and the engine gradually looses power.

The problem is that this is an intimidated problem and doesn’t happen all the time. its seems to happen when i travel long distance, thus it might be a heating problem where something over heats.

My car also seems to battle to start when it has been standing for a week or so and when the weather is very hot.

I have search the internet high and low, it seems that there are people with the same problems but no solutions.

I have replaced my fuel pump and fuel filter. the fuel filter were very dirty as it seems that it was the fist time that it has been replace. I have also done a fault reading and no fault was found. I used Injector cleaner about three times thinking that the injectors might be dirty.

I do quite a bit of dirt road driving as well maybe something could just be dirty ? Air filter is replace every 15 000 km

I really hope someone could help me with this problem.