I've got 2 lights in the house that have an intermittent flicker (we can go for days without having it at all, then it'll happen for a bit). I've not seen them do it at the same time. They are on the same circuit (but different switches). It's not much of a flicker really, closer to a very subtle change in light output, just enough to annoy and concern me (my wife didn't even notice until I pointed it out). They are 2 of the highest power consumers in the house, one being 200 watts, the other 180 watts. For comparison, our third highest consumer is 160 watts, is on a different circuit and has never been seen to flicker.
There is also a 60 watt bulb on the same circuit as my flickering lights and on the same switch as the 200 watt fixture. This one never has been observed to flicker (but perhaps I simply can't notice it as the light output is so much lower).
I'm at a loss as to how to narrow this down further. I've observed other light and appliance usage in the house and it doesn't seem to match up with anything. The closest I've gotten to a pattern is that the 200w unit seems most likely to flicker when used between 5-6am and the 180w when used between 8-9pm. I'm not opposed to calling an electrician, but since I can't reproduce it at will, that seems like I'd probably end up throwing a lot of money at it for no results.
I've now been able to isolate as the other light on the circuit has started flickering (very faintly) now. It's only when the lights are on due to both (3-way) switches being "down". If they're both "up", the lights are flicker free. I'll be checking the connections at the switches now. If not the connections, is that failure usually a bad switch or a bad connection wire for the 3-way setup?