Well, dictionary.com is definitely, positively, beyond a shadow of a doubt wrong, because the word "founder" carries the weight of blind panic among horse people. It's a worse case scenario situation. Referring to a stumble as "foundering" would be about the same as referring to a sneeze as cancer, if cancer were to have a verb form, which I'm realizing it does not, therefore spoiling my analogy. But yeah, dictionary.com is very wrong in this.