help needed

hi all after getting my 360 flashed/soft modded yesterday it plays back up games when it wants sometimes the game will load fine other times i get a the disc is unplayable message on the screen.can i point out that 4 months ago it was again flashed and i had the 3 red lights come on so it went back to microsoft and was returned with new motherboard dvd rom and whatever else when i got it back it was no longer flashed but worked all originals perfect.it still playus originals everytime just the back up ones.when they load they work perfect.as i said some work some dont even on same media type discs.sometimes the same game will load other times it wont.
so is it a flashing error or a dvd rom error any help would be mint cheers all.