Try putting a pressure test on the coolant system first to see if there are any obvious leaks.
If it holds pressure then you could be looking at a blockage. Is there any return flow back to the header tank, try running the car with the cap off when cold to see if the coolant is returning ok to the tank.
Pressure test kits are quite cheap on ebay.
Sundance
The only time you would normally get rapid temp change Indicated is when you have gases in the coolant. When the temp sensor is sensing not coolant but a bubble of gas.
Positive pressure in the crankcase also suggests gases are escaping the combustion chamber and entering oilways somehow. This could be head gasket, worn rings, or cracked liner.
Where in Oz are you? If you're in Melbourne you can bring it around to my place in Burwood, and I'll systematically check over it for you.
Things to check:
Are ALL the fans kicking in? don't forget the engine bay fan. - both fans running fine - even programmed ECU to get fans to come on a bit lower temps to try and help
Have you back flushed the radiator? back flushed all cooling system - removed radiator and checked - its in very good condition.
A cracked liner should show up in a compression leak-down test, a sniffer test should also show this.
A compression test should show a failed head gasket.
Just because parts are new, doesn't mean they're not faulty, check over all the new parts, test that thermostat and check that it is the correct one.
Pull the spark plugs, is one super clean in comparison to the others? Coolant getting into the combustion chamber will often "steam clean" the plug - all plugs look good with light grey. I did notice when cyclinder 4 spark lead was removed less blowby was evident.
I had exactly the same I ran round the houses trying everything to no avail, eventually I had one of our mobile techs Jon from Rough luck to look at it, he inserted a camera into the bores via the plug holes and number 1 cylinder was wet, so over heat was due to gasses although a sniff test said otherwise, I also had a multi layer head gasket that han done less than 4000 miles liners were ok, just the gasket had let go, Jon changed the gasket for an N series gasket and all well 4 years hence.