As far as cleaning DVDs, I've had good luck with the lens cleaning cloths and fluid that you can get at Walmart in the optical section. I apply a liberal amount of fluid to the disk and use a very, very gentle touch, making sure that the part of the cloth that touches the disk has no creases in it. I never press the cloth against the disk with my fingers, but rather bunch up the cloth and just use the natural bulge of the fabric to keep it in contact, again watching out for creases or edges in the cloth. I've never scratched a DVD this way, and they usually come out sparkling.

Some DVDs have a smallprint warning of sorts on the back of the box that when the player switches between layers on the disk, it may cause a slight "hiccup" or delay. I've only seen this happen once. Perhaps that's what you're seeing if you get a glitch somewhere near the middle of the movie...a dirty disk may complicate it, I suppose.