Looking for some input ?

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I have a 2003 H2 and have noticed that the acceleration is not consistant to where you put your foot into the pedal. The truck does not have much mid range, you put the pedal down and down and it will kick down a gear eventually, however does not seem right, no codes have come up and it has run this way for some time, and as i said it is not consistant. At a set of lights there is no hesistation on take off, lots of pickup, only when under load for a period of time during travel.
Any ideas would be appreciated, i have been thinking fuel? maybe a regulator?.

Thanks