first thing you need to make sure of, is that the boom and extend are fully stowed. (boom all the way down and extension section is retracted).
Check the display for any active faults during operation when this condition is happening. (will need someone else to watch it while unit is being driven).
If the engine is laboring in high speed mode only there could be a problem in the 2 speed valve but first things first, check the things i advised on first before jumping to something else prematurely.