My 91 300E had allot of the same symptoms as what you describe. It also had way over 100K miles and the original windshield. The blue-goo will help some, but you have to pressure hose ad nauseam to really get it off. Buffing very aggressivley with a wet terry helps too. I've heard Bon Ami (in the household cleaning aisle of your supermarket) also works well on windshields. One step that made worlds of difference in my experience is the use of the STP "Vision Blade". It takes the old Rain-X idea and goes a genereration further. From what I understand, it is a polymer that bonds to your windshiled that wears itself off in about 6 months. It causes water to bead rather than sheet on the glass. I noticed a bit of difference in that A: I didn't have to use my wiper as much, B: when I did it cleared with virtually no streaking, and C: when I was done washing the car and dragged the chamois or terry over the treated glass it only took one quick pass and was literally 100% dry. The untreated glass took a couple passes or left a film that lingered for a second or two. The stuff costs about ten bucks and really does last months. It is sort of odd to put on, and you have to clean the glass like hell beforehand, but all-in-all it is worth the effort. Give those steps a try before you get a new winshield. I'm not saying it will be perfect, but it should improve significantly from what it is now.

Best of Luck...Lee