Okay, as suggested by , I downgraded to version 4.11.2, created a new third-person project, and did a bunch of stuff for 20 minutes or so with no freezes or other issues with performance.
I then switched to 4.13.1 and ran the same test (created a new third-person project, did some stuff in the editor), and the freezes took place. I noticed that the engine’s performance (ignoring the freezes) was also poorer - for exmple, selecting a piece of geometry caused the engine to freeze for a second, and the engine would freeze repeatedly while moving objects about.
I would therefore suggest that what is causing these freezes is something introduced to the editor after version 4.11.2 - and is not (at least in my case) an issue with hardware or compatibility issues with drivers, etc.