You said you tried new blades. I noticed that mine would do the same thing with different blades. I don't think the blades are exactly the same size as other mowers. Like my 36"s blades were not exactly 18" there were actually like 18 1/16 wide so if I bought blades that were 18" there was an extra 1/8" space between the blades allowing grass to sneak through. Also if you overtime have a tendancy to sharpen your blades more on the outer edge as I tend to you end up doing the same thing.
What do you mean by streaks? You may have a bad spindle or one that is not aligned right. ie. you hit something and it got kicked out of possition a little.
If you dealer can't figure it out, how old is the machine? Tell them to give you a new one if this has been an on going problem, or atleast make them give you a loaner until they do fix it.