Hi Allison,
It is my husband who has antisynthetase syndrome, not me, so I have never been on pred. I am a resistered nurse though, so I am going to jump in here.
I think it is a very bad idea to go up and down on your prednison, especially if you take 20 mg for a couple days and then go all the way back down to 5 mg. Your body does not produce it's own prednisone at all when you take it artificially-in the form of a pill. So you may feel better for the couple days after you take the higher dose, but you will feel way worseby about the second day you only take the 5 mg dose! Also dropping 2.5 mg a month was probably way too fast. My husband's doctors recommend dropping 1 mg every 1-3 months after getting down to 20mg. This gives your body a chance to start producing it's own and allows you to stop the taper when you start feeling worse. You may never be able to get off the predisone. It is a drug with lots of side effects, but I think you have to decide what is worse, the side effects of the pred, or the symptoms it suppresses?
Also, I believe that predisone sparing agents like Cellcept work better if your symptoms are suppressed when you start taking them, so you may want to talk to your doctors about increasing the dose of prednisone (and getting your CPK level down) before you start the Cellcept.
Just my thoughts,
Bev