Hey Mike!
In my experience IVIG can wear off more quickly with an infection. My first infusion acted the same as yours. Felt amazing for 2-3 weeks then quickly started to feel worse again. For me the IVIG has lost much of it's effect and is used to keep me from backsliding. As for my current symptoms and fluctuations, it doesn't do anything. I get 65g over two days once a month. The longer I stick to my scheduled 4 weeks the more stable I feel. If I do 6 weeks out or miss a month due to schedule conflicts ( there is only one nurse available for my infusions and the clinic is much too far away) I backslide so quickly, usually takes about three months to get back to "normal". And remember many many many things can set off the MG, may not even b that the IVIG is fading, but that your in a small flare? Hopefully you will find a treatment plan that helps stabilize your symptoms soon! And I hope this finds you strong and well.
Jess