DrivingQuant

I have a 2002 Boxster that enjoys dumping the entire contents of the engine coolant on a regular basis. The entire coolant tank seems to be dumped from the emergency overflow whenever this happens. No warning lights appear on the dash either before or after this occurs. The dump happens only when I turn the engine off after driving. After turning the engine off I hear rapid running liquid coming out of the back of the car, and can see the coolant fluid flooding over the pavement. It completely empties the tank and takes about 30 seconds. There does not appear to be a leak in the system. The first time it happened I called roadside assistance they pressure tested the tank without issue. I subsequently took it to a garage which also pressure tested the tank and did not find an issue. The level does not gradually decline: it is either fine or it dumps the whole thing on power-down. A few months ago the car started dumping the coolant very frequently (almost every time I drove). I took it back to the garage which decided to replace the coolant overflow reservoir which apparently had a crack. This seemed to improve the situation, but in the last few weeks the problem has returned and the coolant is dumped after most trips when I turn-off the engine. The engine temperature shown on the dash is normal, no warning-lights come on, and the fans work normally. At least one of the times this has happened in the past I did not notice it immediately and ended up driving around for some time before realising there was no coolant. I have owned the car for about 2 years, and the issue first occurred about a year ago. Has anyone come across this before? I don't really want to take the car back to the garage for them to do yet another random "fix", charge me a fortune and not actually address the problem.