Hiya,
I am a long time lurker on this site and saw your post this morning. I have also been investigated for MG the jury is still out ( everyone except the neurologists believe I have it).
My ears pricked up when you mentioned tachycardia please take a look at this website
http://www.dinet.org/ and this website (part of the same one its a forum)
http://forums.dinet.org/
It maybe possible you have postural orthostatic tacycardia syndrome. On the first link its gives you symptoms and tells you how to perform a poor mans tilt table test.
Please have a look and dont give up seraching for a diagnosis. It took me 37 years to get diagnosed with ehlers danlos syndrome a genetic condition that I had so many red flags for I was diagnosed within ten minutes of meeting the rhuematologist and 4 years to be diagnosed with POTS (postural orthostatic tachycardia syndrome).
I hope this helps
Rach