Well, you certainly haven't lacked in a full work up. The only test I don't see that I think would be worthy is a gastric empty study. With your GI complaints and autonomic dysfunction, gastroparesis (delayed gastric emptying) seems possible and would explain all the GI symptoms.

How's your BP? With the lightheadedness and dizziness, it would be good to check your BP during these episodes. Drops upon standing is consistent with the POTS and would explain the tachy rates as the heart tries to compensate for drops in BP. Treatment options are many to control this and alleviate part of your problems.

Of course, what's causing all this is unknown but Lyme can certainly present with autonomic dysfunction and SFN. With your positive tests this should be considered as the culprit. Autoimmune disease is known to cause false positive lyme tests but your autoimmune work up looks to be thorough. The only thing I don't see testing for is Sjogren's, but you don't mention any dry mouth, eye, fatigue, or joint pain.

BTW, a month of Doxy isn't very long. I had a friend (a farmer who was in great shape) that was in bed for two years with Lyme. Not that this will be your outcome, but Lyme can be bad and cause all kinds of symptoms without rhyme or reason.

While I wait for your reply, I'll search a little to see what I can find on Lyme attacking the DRG. I'd bet it's possible, I have just haven't seen it spelled out in writing yet.