Two or three years between blackouts

Hi chaps, for no apparent reason (I've had loads of tests) I sometimes wake up on the floor, twice whilst sleeping and once when sitting and awake, witnesses said I just dropped and "came to" immediately ... the odd thing about these incidents is they happen only every two or three years ie I can go for a couple of years feeling totally well and then 'bang' ... then another two or three years ... so what illness can be this weird ?