Try this:
1. Download MSI Afterburner here http://event.msi.com/vga/afterburner/download.htm
2. Install it and run it.
3. In the "menu" there is a "power limit" slider. Put the slider to lower value (70 - 80%).
4. That's all! This limitation prevents Nvidia BOOST - does not undervolt. core.
Hope it helps.
Dan