Last summer, I installed an incandescent dimmer switch (max 600 W) for my ceiling fan light fixture, which contains four 40 W incandescent bulbs. It worked great for ~10 months at all brightness levels. But this summer, I started having problems with the bulbs flickering intermittently and/or going out after a period of time. If increased to full brightness, they go out immediately. These are still the original 40W bulbs; I've never changed them.

Because this only became an issue gradually, many months after installation, I assumed it was a loose wire connection. I took everything apart and reconnected it, but no change. When the dimmer is swapped out for a standard on/off switch, the light works just fine with no flickering. I thought it must be a problem with the dimmer switch and bought a new one, but observed the same problem with the replacement dimmer switch.

To make this even more confusing, I have an identical fan and dimmer switch set up in another bedroom. They were installed in the same week, but the dimmer in the other room has never exhibited this problem...

I'm stumped. I've found many threads which mention this problem for LED light/dimmer setups (usually attributed to compatibility), but my setup is for incandescents and does not involve LEDs. I've gone back to the standard switch for now, but would love to know what's going wrong with dimmer.