My Alto Saxophone keeps squealing in crazy harmonics
I recently switched to a Yamaha Custom EX Alto Saxophone and at first, it was a dream. Before that I had a crappy YAS-270 from when I first started (7 years), and I thought that after 7 years of playing I definitely needed to step up from a beginner instrument that had leaking pads. When I first played the EX, the sound quality was amazing beyond comparison and every note was crisp and came out smoothly. Fast forward a month or so to now, I cant even get a high D out of my instrument without it squealing some note 3 octaves above what is played... It is definitely not my mouthpiece, as I tried it back on my old YAS-270 and it was absolutely fine on the high notes.
I got this saxophone second hand in mint condition for $3800 when it is usually $5600.
Can someone please give me an idea as to why it is doing this? I have checked, and there are no leaking pads and the neck is secure to the body when screwed on. I have tried loosening my embouchure a lot, like I'm playing the Bari and it helps a bit but still squeals like all 5 hells have been unleashed...
I've got my grade 6 exam coming up so I hope I can get it fixed by then. If I can't get help from here I plan to take it in for a service.