About the switch
In the UK dimmer switches with modules that look like this are quite popular: https://ssli.ebayimg.com/images/g/7f0AAOSwMvtZOrsC/s-l640.jpg It's a mains light dimmer, usually rated 50W-250W. Having searched the web a bit and disassembled a few, they appear to be triac dimmers.
I use these dimmers in rooms with three to five lightbulbs. I have recently replaced the lightbulbs with dimmable LEDs, leaving one halogen in, so load is still 50W+. Without that they flicker when dimmed (my understanding is that triacs have a minimum current requirement). All fine so far.
A dimmer switch broke, so I got a replacement.
It worked fine for a few hours, then lights started flickering (my guess is 10-20Hz) and were a bit dimmer. Sometimes flickering stops for a bit, lights get brighter, then starts again etc. IOW, behaviour changes over time, but is never fine.
Switching off/on doesn't help. Dimmer was dialed to full power when this happened. This has happened in multiple rooms (I've moved dimmer switches around to eliminate other factors), and with three replacement dimmer switches. With one of them it took a few days for this fault to develop. After fault develops, that dimmer switch does not work correctly anymore.
Now, the fix seems obvious: source dimmers elsewhere. But I feel uncomfortable not knowing the root cause, and I think this could be an learning experience.
What is happening?
It appears I'm using these dimmers in some incorrect way, although that's not obvious to me. I guess circuit in LEDs could draw too much current through triac in dimmer, but that theory seems weird.