I first starting having symptoms like this about 7 years ago. The only thing that turned up in my diagnostics was a high hematocrit(56). If I spoke, I got terribly dizzy, but only within a few hours of having had a meal. On an empty stomach I did much better. Well, I found the reason why my hematocrit was high, was able to correct it, and my symptoms went away. Now several years later my hematocrit is normal but the symptoms are back. I can only assume the problem is vascular in nature at this point. More diagnostics pending but nothing has turned up thus far. I strongly suspect that tension in my neck is cutting off the blood supply to my brain while I talk. I’ll let you know if I solve my mystery.