Assuming that you're using an up to date driver, AMD has recently changed its overclocking tool to WattMan. Haven't seen in it action (currently using NVIDIA at work and my home cards are from the 5xxx generation), but I've read that it had some problems initially.
First of all,
On the PC front, I'd suggest doing a reset in WattMan, if there's a way, just to make sure nothing is screwy. If that doesn't help, perhaps defaults are wrong, trying playing with it. One thing I'd suggest is undervolting, that is, reducing voltage. AMD cards and CPU's often work fine with lower voltage, and that reduces power consumption and heat.
If WattMan isn't available for the 280x, Try MSI Afterburner.