to test a stable clock ,better use MotherNature Scene with all on max settings over Night with a loop! Its more near by realtity! Disable all Case Cooling Fans (if possible with FanControl)..to have only the real heat from the GPU...
Artifact Scan is very nice, but for long loops (+5h) i suggest you use some thing else like MotherNature Loop! Cause there can be some Erros in Rendring Code of Artifact Scan that will give you Errors after few hours...MotherNature gives you only a Error "when there is a real Error"...
On MotherNature Loop you can use RivaTuners TempMontitoring..so you've the best possible overview on your testing Run to find out Errors cause of to high Temps or somewhat!