Copper water pipe may or may not be a great ground, there could be a section of plastic pipe, a connector, maybe a water filter or water softener or who knows what in line before it can make it outside and be a decent potential grounding point. The only grounding point that is important is the one at the power panel. Now, you often bond things like the metal pipes to that ground, but generally, you should not rely on them as your grounding point. Even the soil may not be a very good grounding point, depending on its composition, the moisture content, and probably other factors. For example, dry sand makes a lousy ground point. Way back when I was in the military, on field exercises in the desert, they would requisition large quantities of table salt, mix that into the soil where they needed a good ground, and keep it wet. This improved both the electrical safety and the performance of things like antennae, since it then provided a decent ground plane as well. It can sometimes take numerous ground rods to get a decent ground in parallel to get the resistance down. Pull the breakers out of the panel and look at the bus bar and their contacts. If they come out easily, over time, the heating/cooling from changes in load may have made their spring contact pressure too low to maintain a good connection. There could be some arcing or corrosion there as well.
Have you checked the connections in the main panel for tightness, arcing, or corrosion? Sometimes, just a loose connection can be a source of all sorts of weird indications.