cgminer will automatically reduce the frequency if the asic miner plateaus in order to protect it. This usually happens due to overheating or low voltage.

One of my miners gets adjusted on a regular basis and I've spent a lot of time trying to get it to stay at 400. All the others stay at 400 just fine. I've had them all at 400 before, but it just comes down to how much you want to troubleshoot.