I bought a model of a t-rex a while back and have spent quite a few days trying to fix what i believe to be a weight painting problem that came with the model when i bought it and have come up with nothing. 
I have next to no experience when it comes to weight painting!
I cannot pay you! so If you are willing to help me in your spare time to get this resolved I would be very grateful. If you are interested please let me know and i will pm the blend file to you
(providing someone hasn’t offered!)
Thank you,
Blenderscool09