Triton slow to accelerate

We have a recurring problem with our 2007 Mitsubishi Triton. When putting it into first gear at a set of lights we are able to move forward, but we lose all power and cannot rev higher than 2000 rpm even with your foot flat on the accelerator, and it will very slowly get up to speed. Once in third and higher the car seems normal. We have had numerous mechanics look at the car with no luck. We initially thought it was the vacuum hose, but after replacing the hose it is still happening? Can you shed any light on to what might be going on and how we can fix it?