I'm afraid I don't have much knowledge of medication such as methadone, but I can relate to the jerking certainly.
Can you describe it anymore? When I jerk it seems like it is every muscle in my body, but yet ends up affecting my joints as well because it is so violent.
How long does it last?
My jerking seems to be epilepsy related, but yet sometimes the pain occurs independently, far more RLS related.
What pain is it that you take the medication for? Is it muscular or joint pain?
I hope you don't mind all the questions, I am curious!