This is getting curiouser and curiouser.
I still seem to be getting the distortion problem after a period of use on normal output, but the blue output buffer indicator on the "sound cards" tab now stays rock steady at 50%. Clicking on "restart" still solves the problem instantly. I turned on the auto-restart feature but I left the settings for it at the default of 0% and 125% as I dont know what they do. It seems to me that a better figure for the "restart if above" setting would be around 95%, if indeed this is what is causing my problem. 125% seems too high to me, but what do I know?
I wonder if this could possibly be something to do with playing source material that has different bit rates? It seems to come on faster when source bit rates change, but it's very hard to tell when it creeps up over a period of many minutes like this.
I still dont know if it's best to select WASAPI or MME or Kernel Streaming for the input/output. They dont all seem to work all of the time, though they do all seem to work some of the time. Very confusing.