Mitsubishi Outlander loses power

Hi,

I have a 2003 Mitsubishi Outlander XLS (2.4 AWD) with 35,000 miles.

When I depress the accelerator pedal the vehicle sometimes loses power (I discovered the problem only while attempting to overtake a vehicle on a narrow road!). What is certain is that there is very little power (5-10 miles per hour) when I climb a very steep gradient (resulting in the check engine light illuminating intermittently as the strain increases or decreases on the engine - sometimes with a slight backfire). Sometimes though I can hit top speed without much problem driving on regular roads. Owing to the fact that this happened within 4 minutes of having bought fuel, I assumed there was something wrong with the gas and had the gas tank removed and emptied. When this did not help I changed the in-tank fuel filter, the spark plugs, the air filter and the PCV valve (and the oil and oil filter for good measure). With each change the vehicle's performance has improved for driving on non-steep roads, however, I am still having the problem (more so on steep gradients).

Need your help on this one, thanks