are you over heating? using loads of water, any smoke/steam from exhaust?

have you compression tested it?

also you could have a sniffer test done on it, this is where a bottle is put on ya header tank and a fluid is put into it, then it sucks the fumes from the header tank and if exhaust gas is getting into the water the fluid will change colour.

i had one done ages ago, got ripped a bit as it was the only place on the isle of wight that couls do it that day and it cost me £20, £20 for nothing as it was ok but your prob only pay a tenner for it or so.

might be worth getting done before ripping the head off.