Random font size changes in Editor

I’ve been using Scrivener on almost daily basis for some months now without any problems that couldn’t be put down to my own lack of knowledge of the program. But something’s started happening in the past few days that has me foxed. The text size in the Editor often changes randomly, and not as a whole - some paragraphs will have larger font sizes than others, and sometimes these sizes will be very large - maybe 30 point or more. They might, on the other hand, be tiny, and more than once I’ve had a startling mixture of sizes showing on screen.

However, the global size, as selected in the bar at the bottom of the screen won’t have changed (I keep it at 175%), and neither will the details on the top bar (mine are always Garamond 12).

Usually, the only way to correct this is to wiggle the slider on the right (I don’t know the technical terms for these and other things), or switch to another folder and back, but it seems to be getting increasingly recalcitrant.

For the record, I’m using Scrivener 1.9.9.0 with Windows 7.

What might be causing the behaviour I described?

Gareth