Mick,

You really diddn't tell us enough about your process to provide solid answers.

But my gut says that the electrodes are not the problem... 

Tungsten inclusions come from two main possible reasons
   1.  Tungsten is overheated and small droplets transfer across the arc into the weld
   2.  Welder stabbs electrode into the work.

Cerium carries as much current as thorium and lanthanum and more than zirconium or pure; So your not going to find a doping alloy that provides more capacity than cerium.

Are you working with Alternating Current and aluminum or DC and steels?   Knowing this would go a long way to providing help.

Generally speaking if you are getting enough tungsten in your inclusion to fail an RT (in most codes) you have dropped in a substantial amount...  This is almost always attributable to poor operator performance...   Either an electrode diameter is selected that is not rated for the current being used.  Or the electrode is simply poked into the work and the operator did not bother to stop and remove the inclusion before finishing out the weld.

A bit more info may help.  Are you doing manual GTAW?  Preparation of the tungsten is the most important item.  The taper should be ground in the long direction and the end should be ground with a flat.  Grinding marks around the tungsten will cause stress risers and without a flat, the current density at the point can be extremely high, causing the end to pop off.  Back in my days as a welder in the Navy nuclear program, all tungsten grinding was done, for the welders, by the tool shop.  The tungstens were polished to a very fine finish and inspected with fluorescent PT to insure no surface defects.

Inclusions caused by the welder dipping the tungsten into the molten pool are a matter of technique.  The type of tungsten has little to do with the problem.  Proper end prep and a adequate diameter for the amperage level being used will go a long way to helping eliminate the problem.  Good luck.