Overheating

[igosaur]

Imagine the scene. I'm driving along the motorway at about 80 m.p.h. and start to notice the temp gauge going up a bit.

"That's ok", I think, "after all, I don't normally go this fast"!

However, it didn't just go up a bit higher than normal, it went all the way and at some points it was even nudging the red.

When I arrived at my destination I checked and discovered that the engine seemed to have dumped the majority of the coolant into the coolant reservoir instead of keeping it circulating around the engine and radiator where it was needed!

Also, I had to wait at least half an hour before the car would start again even though the temp gauge by this time was reading normal.

Now, this had also happened several months ago but I thought I had fixed it by changing the thermostat and repairing a split hose. But now it's done it again.

What problem could be causing this strange effect? It shouldn't be the thermostat because that is new, and as far as I can tell I don't have a leak.

Any ideas?