Thank you both for your responses. They were incredibly helpful. Looking back at my notes for the events I've experienced, the majority did occur while I was standing or exerting myself (tidying up the house, cooking) although a few happened while sitting. They match the descriptions you give of presyncope so well. The other night, after feeling that spacey sensation I actually did feel as though I was going to pass out. I wasn't sure what exactly was happening (seizure or POTS-related symptom), but it was quite frightening. I will definitely bring this information up with my new neurologist when I meet him in a few weeks. Thank you, again, for sharing this information and your experiences with me!