2000 Isuzu Rodeo Question spark plug misfire - random ???

6 months ago our 2000 rodeo threw a misfire code... took it to the mechanic (local guy) he pulled the plugs and found water in the cylinders...head gaskets....replaced gaskets and rebuilt both heads and all ok... for two months,

another plug code... replace bad plug all ok for two months... happened again and he can't figure it out... changed the bad plug and all ok for a couple months ...

now I'm getting a 'random spark plug misfire code...car idles rough...misses at highway speed... but get this... not all the time !!! sometimes it runs great ?????

any suggestions from your isuzu gurus