Everything that you mentioned that you are experiencing are all symptoms of TMJ, sadly. It's pretty amazing and fascinating to study but it is not so nice to be a TMJ sufferer. Mainly because doctors just don't seem to care or even want to know about TMJ. There are some good docs out there, though.
The metallic taste in your mouth is a common complaint and can be due to a variety of causes - from medication you may be taking to dental problems.
Since you have seen all these different doctors, etc., I won't recommend that you see a doctor to rule out any undiagnosed health issues. Has your dentist checks you for gum disease? That can be a source.
Not drinking enough water can contribute to problems with taste. Increase your intake and see if it helps.
Medications that can cause metallic tastes are some antibiotics, prenatal vitamins, some anti-depressants, as well as drugs used to treat high blood pressure, rheumatoid arthritis and kidney stones.
I got most of this information from mayoclinic.com and other Internet sites. All also highly recommended proper oral hygiene with regular brushing and (e-ewe) scraping your tongue.
One site even suggested seeing a practitioner of Chinese medicine. O.....K..... I personally wouldn't go to the last, probably (and that could be a mistake) and I don't like the scraping of the tongue. It just grosses me out. But the things we will do to help our bodies can be pretty amazing sometimes.
I hope this helps. I pick a medication as being the culprit, but I also don't know what you take, if anything. I am a little surprised (or maybe I'm not) that your doctors/dentists wouldn't take you seriously. I really have a difficult time "liking" the medical profession after all I have been through these last 27 years of TMJ. But now I am doing 90% better and I do have doctors to thank for that, but it sure took some persuasion on my part for them to listen to me sometimes.
Keep up the research and continue to look for doctors and dentists who will listen to you and at least ---- try to help.
Sorry for my many words but that's just me. Wordy. Please take care and let us know how you are doing.
God bless....
Carol