I suspect the lamp as well.
I'm not sure on your bulb warranty, so check to see if it's covered first.
If not, then you can try the following.
Cycle the lamp from low power to high over the space of an hour. Leave about 5mins between each cycle.
Stop at any point, if the lamp returns to normal brightness.
Sometimes the metallic vapour in the lamp can cluster on the element.
Cycling can sometimes help by releasing these particles.
Please note there is a risk that this may actually totally kill a bulb already on it's way!
Since it sounds as if it's on the way out anyway, you may have nothing to lose.
Try at your own risk etc...