Some code and bonding questions. I am troubleshooting flickering lights in a large historic home. Renovated 3-years ago. We reviewed the electrical distribution, which has lead to some controversy on grounding and bonding. Served by a 50kVA 120/240 split single phase Xmer. Shared by three homes. All phase conductors from Xmer to house main panel are 4/0 AL. All Neutrals are 2/0 AL. 150’ overhead run from Xmer to next pole, then underground, across alley, to meter and 200amp. outdoor panel on detached garage. Then 150’ buried 3-wire run to 200amp. disconnect located in yard. Then 100’ buried 4-wire run to home main panel. Then two sub-panels. Both outdoor panel and disconnect have two #6 bare wire grounds originating from underground. Both have neutral-ground bonds. We see about a 5-volt drop when large 120-volt home loads turn on. We see a corresponding 2-volt surge on the opposite phase. I have seen voltage readings on loose and missing bonds. Usually more severe voltage shifts. The main copper cold water pipe is right by the main house panel. Why would they put disconnect in middle of yard? Is it legal to bond both outdoor detached garage panel and disconnect? Can we move the disconnect inside home and bond using copper pipe as one GEC? Will poor existing grounds explain voltage shift with 120-volt loads? Other suggestions? Thank you.