You should wait a month or so for the new AMD Radeons.
Verify you have the most current BIOS. At the main BIOS screen press <Ctrl> + <F1> for expanded menu options.
1) Set the Memory Clock to *Manual*
2) Drop the Memory Clock setting back one speed
3) Lock the PCIe to 100MHz
Maximum vCore per AMD spec is 1.4v. At 240MHz base clock you should be running 3.48GHz with your RAMs at spec. Max temps at load should not exceed 74c.
It is not uncommon for GB AMD chipsets to hit 280MHz base clock with good cooling --- not sure about your chipset. If yah want to give it a crank above 240MHz you should drop your memory clock back another notch.
Here's my unlocked Sempy on a GB 785g (using an old Opty copper-heatpipe cooler) -- didn't break a sweat but I didn't go higher because of my lack of FET cooling ...
. Grab OCCT
for stress testing.