jd / Jake, that makes sense. I've been thinking about this and I'm going to get the oil cooler and water pump inspected / replaced and then flush the coolant system / buy parts as required. I think it's wise to replace these common failures points rather than simply flush and see, since I'll likely have to flush the coolant with a new pump fitted. Does this make sense?

Hi I have the same problem with my 1999 boxster 97k miles and I am in the process of draining and flushing the coolant but what you will find is that the gunge stickes to the hoses and getting it out of the heater and the radiators is a nightmare. I know that i will be frowned upon by some people with regards to my method of cleaning but it seems to be working. I start by removing the thermostat also disconnect the vent pipe under the car I then placed a dishwasher tablet in the expansion bottle and kept it full with an hose pipe. and repeated this about 5 times until the water was running clear. Also make sure the vent pipe that goes up to the expansion bottle is not full of that gunge which mine was. If that is the case you would never be able to vent off the air whan reffilling hope that helps regards Bill