Couple of suggestions

1) Cant remember exactly but crank angle sender (should throw a sporadic though.)
2) Coil Pack dying slowly. (that might explain no faults)

Possible (but not that likely tbh)

Cam chain loosing tension. (thats a gearbox out job)
Crappy earth's

Have you done a Compression check yet? As its intermittant it would sound more likely (though of course not 100%) to be an electrical issue.