The first suspect for me is the valves, maby 1 has a burnt seat and is not sealing properly.
To check this repeat the compression test then put some oil down each plug hole (a teaspoon or 2 should do it) then do the compression test again, the oil makes a very good temporary seal on the rings and if it dosen't heavely affect the results it rules them out)
i think the compression tester uses a rubber 'O' ring so it will seal on a slightly rough surfice.
But from what you have said i would check the valve clearances first, if the clearance is too tight the valve will not close fully when the engine is warm and this will damage the valve and seat.
