Low revs,EML

Hi all I was wondering if anyone could help me. We have a 1.8 04 plate freelander had head gasket done 10 months ago and new could plugs and a O2 sensor. Recently it started losing water not externally and after a couple of hours standing or in morning it would splutter and miss for a short time. Concluded inlet manifold gasket so I replaced it and put new spark plugs in and bled it up all good heating works ter than it ever did. NOW after 15 mins of driving revs drop to 500rpm engine light flashes and she starts to misfire. But of a lose as at idle she will heat up and idle perfectly light doesnt stay on just flashes. Could be an earth fault or is it another O2 issue.