How to detect peaks ignoring soft declivity?
You mentioned that you have tried the signal derivative to catch the declines. What problems have you faced?

It seems that your problem might be addressed with different words: Let $s(t)$ be your signal, and let $u(t)=ds(t)/dt$. You want a detector to indicate when $a\le u(t)\le b$, with $a,b&lt;0$, for at least $T$ seconds.

If this rewrite of the problem is true, the challenge now is to calculate $u(t)$. You can use u=diff(s), as you mentioned if the signal $s(t)$ has no noise, or you need to estimate $u(t)$ using other techniques like these two for instance.