2000 Chevy Blazer voltage drop

When I accelerate fast, or travel at a sustained speed of 70 mph on the highway, or go up a steep hill, (basically anytime I put a load on the engin) the volt meter drops to between the 14 mark and the lower mark of 9. The battery light also comes on. When the RPMs drop back to normal (under 2000) the vots get back to 14, the light goes out. When I turn the key to on without starting the engine the volts read around 12. When I start the engine and let it idle, the volts are at about 14. I tested the battery and it was fully charged and all cells were good. It is only 1 1/2 years old. The alternator is about 2.5 years old. Can you help me with diagnosing the problem.