First, you could insulate your kettle: put a jacket/coat or some other form of insulation around your kettle.
Second, there's always a good chance that the temp in your grain bed will be lower than the liquid surrounding the grains. While heating, make sure you stir every few minutes to keep it even (assuming you're heating your grains and your water together), and also stir a few times during your mash.
Also, do you heat your grain and ALL of your water together? That will help a bit, but if that's not what you do, and your kettle is already too small, it might be the time to upgrade to a larger one.