Here's my thoughts on the 'good day'. You state you have GI symptoms 24/7, so I assume they were present during the test, right? If so, then it's obviously not gastroparesis. If your symptoms were not present during either test then I'd guess it would leave that window open...but only slightly, as I would think even a mild delay would be noted on a 'good day' if this was a significant problem. Of course being tested while you're experiencing symptoms is best, but doesn't always work out that way.

Your BP sounds good. Are they treating the POTS and tachy rates?

Sjogren's patients have dry mouth all day, not just upon waking after mouth breathing in the night. Same for dry eyes...foreign body sensation, gritty, and lack of tear production. I missed reading that you had the labs (SSA & SSB). Keep in mind that only 40% of Sjogren's patients have positive labs. The lip biopsy is required by most criteria for confirmation. Sure there have been some with negative procedures. You'd likely be able to find horror stories about any type of biopsy...or even a tooth extraction or ear wax removal, if you're reading this online. I have talked to a couple people that truly had a bad experience, but most did fine. I have a tiny little lump that's numb, but certainly not noticeable or bothersome. I would suggest though (if you choose to have it done) that you do so in a hospital that does many-- by a doctor with good amount of experience. Ask questions about their procedure, biopsy grading scale, and experience...be an informed patient.

As for the urination. Is your input consistent with your output? I don't see a glucose tolerance test (long one) among your list of completed tests. As MrsD mentioned earlier in this thread, impaired glucose can account for increase urination.

Where was the rash you mentioned? Did it itch? How long did it stay?

Sometimes (most times) diagnostics is like a puzzle. You can spend months(if not years) investigating to find a cause...and still not find it in some cases. You cannot expect to get diagnosed online. We are here just to help explore ideas so you can have intelligent and helpful discussions with your doctor so they may get you some answers. I would suggest that you at least seek out treatment for the bothersome symptoms you have while you continue your search for causes. There are a variety of treatments for POTS and many options to help alleviate your GI symptoms. Discuss these with you doctor.