Honda CRX expansion tank problem
 

I have just bought a K reg Mk 3 honda crx. It seems to have a water losing problem. I noticed the other day after a trip up the motorway it was hissing and after lifting the bonnet I notice water spurting out of the top of the expansion tank. I was advised to get a new lid and pipes for it, so i did this and also bought a new tank. This seems to have solved the 'hissing' but after a long drive you can here bubbling and all the water disapears out of the expansion tank, my first thoughts were head gasket, but i can see wet patches below the tank and can't seem to see where the water's coming from, there doesn't appear to be water in the oil either or visa versa.

Anyone come accross this before??

Note to sat, temperature gauge is showing normal temperature, no overheating.

I've just noticed some black smoke coming out of the exhaust when pulling off when cold and also the temperature gauge is being erratic, one minute it's still on cold after driving for 20miles then it'll decided to go up to half way then back to cold....