Problems 420EX or Rebel Ti?

Just a quick question;

Has anybody ever noticed a problem with either the Canon 420EX or the
Rebel Ti (300V) where the flash sync is out in P mode?

By 'out' I mean that the falsh will fire, I can see the flash & shadows
in the viewfinder, THEN the mirror flips up and the photo is taken.

For ref, the shutter speed is 1/90 or 1/60 and I was using F/5.6 the
whole shoot. Out of the roll (36exp) there was 8 shots that have come
out underexposed - each shot is where the flash fired before the mirror
flipped up (I took notes)

any help would be appreciated

cheers

Rob