Thanks for your replies and sorry for the slow response. I'm in full-on edit mode ATM, so will be trying your suggestions as soon as I get a moment.
Just a quick observation - the stuttering also happens when playing a file in Media Explorer and I THINK it also happens when playing a Soundcloud file in a browser. So, not one of my downloaded or exported files or using Reaper at all.
I only THINK coz this is when I'm looking for a suitable music track for the podcast and I'm flicking from one to another and the 2 times I THINK it's happened has been milliseconds before I've clicked to move on to something else.
Will experiment further when I get a chance.