Right grab your self a coffee and get set for a story. 2 weeks ago I purchased a 96 2.0 twin turbo legacy went to have a look a week prior had it running/driving for a wile no problems same again a week later before handing over the cash 20 mins after setting off for home it started to over heat and blow water out the header tank when I got to a mates house (about half way home) we found the thermostat wouldn't open at all so get it going without one to get me home no problems it then stood a week doing nothing in till this weekend replaced the thermostat (genuine part) spent what seamed like days bleeding it up had every pipe running warm heaters red hot ran fine rest off the day. Went to it this morning coolent levels were a bit low topped it up let it have 5 mins or so warming up nipped into town and set off on a run to Newark 20 mins in the temp started climing going to about 3/4 the way up then dropping back down turned round and headed home as I got home the heaters were hot when I parked they went cold as was the bottom hose left ticking over after the temp gauge came to normal they both got warm stayed that way till I drove it same again and it now blows the pipe and lid off the header tank anyone have any ideas what it is other then headgasket before this thing cost me a grand more to fix then I played for it 

Get a sniff test done to check for headgasket faliure , its about the most reliable way to check to see if hydrocarbons are in the coolant .

If there's none . Is the thermostat opening and the rad fans cycling?

Have you tried a new rad or header tank cap ?

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Fans cut in seams to be only thing that keeps it from running off the gauge rad not blocked as flushed it out is a bit rusty at bottom but water tight the cap has no markings or anything on it don't think it's original if I'm honest have got a 0.9 one kicking about could try that one