Sounds to me like you have a problem with your DVD player. Is it under warranty?
Sounds like it's having problems reading the discs properly, could mean that it's laser is misaligned or dirty I guess. Probably best to have it checked out asap.
It is usual (to a lesser or greater extent depending on how much you paid for your player) for graduating areas of colour to look pixelly in some circumstances, but I take it from your description that actual definite blocky areas are coming up on the screen.
One thing to try might be to try a different output mode, i.e. if you're using RGB, try S-Video on the player to see if this makes any difference.
Do the 'spots' happen if you 'rewind' the DVD and then replay the same section again?
I don't beleive that a problem with the scart cable would cause this, it would more likely cause fuzzyness or breakup since the signal is analogue at this point.