S.S.
Oh girl!
I had the same problem! Mine would go up rather high, then bottom out like you described. It does make you feel awful. I ended up in the hospital at 36 weeks because it threw my potassium out of whack, which just made me feel even worse.
I drank lots of Gatorade. I really don't like it, but it worked. I drank the strawberry-orange am flavor. My doctor also put me on modified bed rest and wouldn't let me be alone except to shower. He was afraid I'd blackout and hit my head or something.
I was induced at 38 weeks 1 day after my husband rather forcefully told my doctor "we" were miserable.
I wish you the best of luck.