My rat goes back and forth between seeming sick and well?

It really does sound like a Myco flare up. If she is lethargic, tembling or has lost her appetite, she'll need a vet visit. At times they will go back and forth like you are seeing happen with your own. Do you have a scale? I'd invest in an oz/gram scale and do a weekly weigh-in for your rats. Any weight lost can mean problems. A quick vet visit will put your mind at ease.