[alfettish]

Hi,
I stopped at a service station yesterday and noticed that my coolant was leaking out of the overflow.
Drove home as I was 1 K away and it was boiling like mad.
Filled it back up with less than 4 litres of fresh coolant and it was full (presuming there was no water in there that's less than half its capacity).
Today after driving some more it was boiling again but only after I stopped. Put another 4 litres in and drove home.
Guage reads a bit higher than normal on highways and about 90 degrees celcius when in a bit of traffic.
Anyone know what's going on? I've checked the oil and water and cannot find signs of head gasket problems.
filling with half capacity might signify a blockage or an air lock.
Any idea?