I was on Avonex for a very long time and had similar initial symptoms to you and they don't suddenly diminish. It is important to remember what interferon does. In simple terms, think of it as a signaling agent not as an immunosuppressive. It doesn't wage any wars, it just alerts your immune system to do something. The same thing occurs naturally when you start to get to ache because you are "coming down with a bug". It's a surge of wake up juice for the immune system.
So if you get really bad reactions when others may not, what is it telling you about your underlying health? The message is you have pathogens in your system that can evade your immune system most of the time but not when the interferon signals to go on the attack. I would suggest you see your local doctor and get tested for chlamydias, rickettsias, lyme, mycoplasmas (especially) and anything else he can think of. It's this style of infection that is making the reaction so difficult. If you had no underlying infections then you would barely notice the Avonex. I have been through the whole range of reactions so I speak from first hand experience. Persisting with Avonex will be really hard if you don't use other treatments to clear the underlying pathogens.