Okay,
after some reed adjusting I came out with a really nice reed, just a bit on the easy side, but in tune and stable (I had an hard time stabling the back D… but I’ve successed.) so I played it quite a bit and it sound very nice and in tune, but when played a bit more, I found out that C sharp is very unstable. i have to be very very gentle with the air stream. this is the only note that makes problems now… too much air and the note fall back to C (a few cents sharper than C, actually).
Should I try to make the reed a bit harder? It’s really hard to keep a steady C#, or even worse - jump to C# from other notes.
I’m afraid that after acheiving a good reed, I could mess things up!
I don’t have any direct experience with a breaking C# so take my advice with a grain of salt. However, it sounds like your reed is overscraped. It is common for the back D to break if the upper portion of the reed is too scraped (especially the lips) so I’m thinking that the C# is suffering instead of the D in your case.
Generally, I have found an overscraped reed to produce a flat Cnat, a sharp C# and a flat unstable back D. If this sounds like your reed and it is very easy to blow, it’s probably overscraped. The fact that the problem kicks in when the reed has been warmed up for a bit reinforces this theory, as your reed will be softer when warmed up than when fist played. If your reed lips are less than a combined thickness of 0.5mm, I’d say they are too thin.
Stiffening a soft reed is very doable, but it took me a while to get the hang of doing it well so you’d have to make a judgement call of whether the problem is bad enough to take the risk.
Another thought - maybe there is a leak in the sides of the reed.