Punto rough running question

I own a 1998 Punto 60SX (8 valve engine) which has done 56,000 miles. It has had regular servicing and until this week has run faultlessly.
It has now developed a slight problem and I don't know what to check.
When starting from cold it starts first touch, but runs roughly for about 20 or 30 seconds, as though misfiring. After this it runs fine.
My thoughts are
* change the plugs - they are about due (20,000 miles since last changed)
* clean HT leads & check they are secure
* Perhaps disconnect & reconnect battery to "wipe" ECU memory - could it have become corrupt?
Beyond this I'm stumped. Most fault finding information I have seen refers to rough idling or rough running, but mine runs rough if idling or pulling away then idles and runs OK after a short time.
I'd be very grateful if anyone can suggest a likely cause - especially if it's cheap to fix :-)
Cheers
Rob
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sounds like a head gasget, a few quick checks. does the heater when on hot sometimes go cold then hot again? does the coolant smell a bit like petrol? is there a lot of pressure in the cooling system? if you said yes them its probly the head gasget. btw ive done over 100 of these headgasgets, my 2nd most common rough running fault on 8v puntos.
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running fault on 8v puntos.

Spot on!
It came to a head (!) on Saturday - heater went cold, coolant pressurised and water temp gauge went to max. Rough running must have been water seeping into a cylinder causing no spark until engine warmed enough to dry plug.
Thanks for diagnosis!
Regards
Rob
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this is exactly what happens and in the very worst case's the engine wont even start at all because of hydraulic lock. This has happened on my own punto before :(
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