In MSI Afterburner, there is an option for "Power Limit." Double checking, I only have it set at +9 with my current clock of 960/1440. But yeah, when the clock bounces like that when at 100% GPU use, it's certainly low wattage. Upping this setting helps make sure it gets more.
My stock clocks were bouncy, too. Only have a 620w.
Absolutely. Increase the power play feature where it ramps up voltage based on load. The top end is not pushing power hard enough.
I've found that you can set it either as a cgminer command (--powertune) or in afterburner. In my case, I actually end up flashing the firmware with the new settings since I run in ESXi where I can't access those in a VM.