Mitsubishi Repair: Eclipse 94 2.0 Non-Turbo manual, is not running as it schould.., straight road, main roads

Question
When i am driving on a straight road or i am going upwards into a hill, my car will not accelerate for some reason, when driving downhill it runs smooth. I have had it in and out of different workshops, but they can not figure out what is wrong. It is at its worst when it is cold outside, and the engine is cold. I am having big trouble now, when driving at the main roads i am the one slowing down the traffic, and a sports car should not act that way...
If you could help me, i would be very grateful.

Answer
Kaare,
This is most likely an air intake problem or a fuel problem.  My guess is that your car is struggling to get fuel to the engine; most likely caused by a weak fuel pump or clogged fuel filter.  You may just need a general tune-up to bring the performance back to your car.  
Good luck!