According to lee h :
franz frippl wrote:

Dimmer switches are no more than rheostats. If carbon were to build
up on part of the switch, it may be sufficient to affect voltage.
Might cause a voltage drop.

You could test this with a volt/ohm meter.

In ye olde days, you were right. However for many years,
the triac-based dimmer switch has been the more common.

"More common"? Even in ye olde days, rheostat dimmers were extremely
rare, and they're now essentially non-existant.

Hint: rheostats have to dissipate a _lot_ of power, and a switch
box is not a good place to try doing that.

A rheostat capable of, say, dimming a 100W light bulb probably
wouldn't fit into a switch box, and if it did, you could use the
wall around it as a cooking surface.

Dimmers tend to fail in one of two ways:

1) Complete failure, no light at any position.
2) triac control fails, get erratic behaviour.

I suspect that the OP is simply seeing a brownout.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.