Clunk when driving fifty five to sixty mph

You need to describe exactly what you are feeling or hearing. What can you do to make it occur or not occur? See if this still happens when you have the heater/AC system turned off. If it does not happen then, you are likely feeling the thump when the AC compressor cycles on. The compressor runs in defrost mode too, so you may feel it in cold weather. If that appears to be the cause, re-centering an engine mount might help. For some models there was a revised engine mount to eliminate that clunk.

Dandy. The clunking was confusing. Intermittent stalling or cutting out sheds a new light on the story. Your dealer is right about waiting for the problem to get worse. There is no defect to be found when the engine is running right, so test-driving the car to try to make that happen would be a waste of your money and their time.

What they might consider is driving the car with a scanner connected so live data can be viewed in real time. Most scanners have a "record" capability that lets it record a few seconds of sensor data that can be replayed slowly, later, to see what changed when he problem occurred. We also used to have a flight recorder that could be sent with the owner. It had a button to press when the problem occurred. In both cases, the data passes through the unit's memory, so the recording actually started a couple of seconds before the button was pressed.

Of particular interest will be the camshaft position sensor and the crankshaft position sensor. If the signal from either one cuts out, the engine computer will not know when to fire the ignition coils and injectors, so stalling will occur. The camshaft position sensor has been known to cut out intermittently.