I am having the same problem as you. I had my gallbladder out in January and I'm still having "gall bladder attacks". My doctor is performing ercp next week. He thinks that there is a stone in the bile duct. Yours sounds tricky because they can't perform this. I wish I could be of some help. Keep us posted.
A MRCP is very useful in detecting stones. If it reveals there are no stones, there is another test called Hida Scan with morphine. It is basically used to see if bile is flowing as it should or if there is a blockage or spasms in the biliary tree. I had my gallbladder removed 2 1/2 years ago and still have a lot of pain in my RUQ. I had a MRCP and the Hida Scan last fall which showed no stones, but the Hida revealed that bile was moving very sluggishly and was diagnosed with sphincter of oddi dysfunction. I also can't have an ERCP done due to having the lower third of my stomach removed and a bypass for unrelated GI problems. I hope this information is helpful.