The ETRS uses leaf shutter lenses, with electrical timing in the camera body, so if it is the shutter sticking then chances are it is the lens not the body. One way to test would be remove the batteries and see whether the mechanically timed 1/500s shutter speed is working properly. If 1/500s is sticking then it's definitely the lens that is faulty.
I haven't tried them, but I believe kohscamera (http://kohscamera.com/) are Bronica specialists and I have read good reports of them.
Congrats on the ETRS, it's a lovely camera system with excellent lenses, I'm sure you will enjoy it once you've solved this problem.
ETRSi, Waist-Level, 40mm f/4 PE and 75 f/2.8 PE lenses.