When confronted with this problem in CFS and Fibromyalgia patients, we found that chronic central nervous system infections were present that could explain the problem. Such infections often go along with short-term
memory loss, so-called brain fog, intermitent confusion, depression, fatigue and other other symptoms.
I would try to assess whether this is only a speech problem, or whether the speech problem is just one of many other problems not expressed in your question.
Prof.