Trouble with squeaking on Saxophone?

I play alto sax and lately, I've been experiencing a lot of squeaking when I go into my highest D, E, and F. Occasionally my middle D will squeak once in a while too. I haven't been noticing this since I opened a new box of reeds (Vandoren 2 1/2). I soaked my reed in my mouth about 20 minutes prior to playing on it the first time, but after just hearing a bunch of squeaks I kept it in longer. I never experienced this problem with my older reed (same brand and strength). I use a Vandoren A27 mouthpiece and a Rovner ligature, if that matters. What can I do to stop it from squeaking?