Depending on the fuel cap and pressure relief, at high altitude and low power there might not be enough fuel flow to keep air flowing into the tanks which can cause a stoppage of the fuel to the engine. If you use one tank at a time the fuel flow from the single tank is greater and will allow for uninterrupted flow. Also another reason why you switch to one tank following engine failure, since if this was the reason it will restore fuel flow and therefore power. There used to be a warning on this in the POH.