I have a Macbook Air with an external mouse. This affects both the mouse and the trackpad, which suggests a software rather than a hardware problem.
OS X Yosemite (10.10.4)
>System Preferences...>Mouse
And... what am I supposed to do there?
I assume that "double-click speed" refers to how long you have before the system will detect two successive clicks as two single clicks instead of one double click. But not to how often the system will mis-detect one click as one double click.
if the interval is too short it's likely the double click is being registered as a single click (each time) - set the interval to a shorter setting and test. If you think this is a software issue this is where you would validate that theory. Otherwise disconnect the mouse and see if it happens with just the trackpad, the mouse could be faulty.
If it works sometimes and not others it's likely a hardware issue and you should probably clean the trackpad
How to disinfect the Apple internal or external keyboard, trackpad, and mouse - Apple Support
"if the interval is too short it's likely the double click is being registered as a single click (each time)"
but the problem is that the single click is being registered as a double click, as in "Yosemite Keeps Mistaking Single Clicks for Double Clicks."
That said, the problem comes and goes, but seems worse with the mouse than with the trackpad. I may just need a new mouse as the old one wears out.
Yosemite Keeps Mistaking Single Clicks for Double Clicks
