On three of my cars (two TTs and an Avant), I get intermittent car reports that brake lights or turn signal lights are not working. On a 2 hour trip, the lights can "fail" and then become OK and then "fail" again every 20 minutes or so.

I have checked the bulbs and the light assembly, and they appear to be OK.

I suspect an intermittent connection either in the light assembly itself, or in the connector from the wiring harness to the light assembly.

Are these kinds of problems common and well understood?