Do TRIAC dimmers "wear out"?
TRIACs do tend to wear out.  As they age, the waveform becomes more jagged.  Here is what a good TRIAC should do to the waveform when 50% dimmed:



Here is what a failing TRIAC might look like:



(That is actually the current of a CFL:  I couldn't find a failing TRIAC waveform.)

If it is failing, TRIACs seem to fail pretty quickly and enter an intermediate poorly-performing state for only a few hours or days.

You could unscrew one or two bulbs in the fixture and see if that improves the situation to isolate whether it is the bulbs or the dimmer.

For more details about dimmers, see this.