SOunds like it could be TOS related. One of the TOS forum members, Dabbo, also was lifting losts of weight and has experienced some of the same problems.

I am thinking nerve entrapment in cervical area or brachial plexus (ie TOS - thoracic outlet syndrome and then the muscles get weak and fatigues. Its a crush or compression injury that can be especially common in wieght lifters and swimmers. Dabbo hurt himself through weight lifting.

Might want to see nuerologist and have nerve tests or imaging done. That might tell where some of the damage is.

Also take a read of some of the posts in the TOS forum particularly from Towelhorse.