If it alternates between working and not working with the time of day, then yes I would suspect the dimmer is the problem. However, I would check well that the connections at the dimmer are secure before replacing the dimmer. The behavior could be explained by a lose connection there as well.

Thanks, ronnatalie. All connections were checked/rechecked - everything seems fine and the lights are working again. We will continue to watch these switches and if intermittent behavior starts again - will start replacing them. Do you think that the dimmer switch is the most likely culprit? Can this be due to faulty 3-way and 4-way switches we have by the door and on the wall? I was thinking start replacing with those two.

Anything is possible, but heat problems more likely will affect an electronic component rather than purely mechanical switches.