I can't agree with that. At start-up the card isn't being fully utilized and they can and do start-up an work with low power (2D mode). It is not until a game/program activates 3D mode that the card really starts consuming it's Full Power requirements.
But I'm wondering what the card's temperatures are. It sounds more like the card is just starting to overheat. Like it's running right on the line of "ok" and "not ok". I would remove the card and remove it's heatsink. Clean the card and the heatsink with compressed air. Clean the actual gpu with alcohol, let it dry and add new thermal compound. Then Re-assemble the card and see if it's back to normal.