You are right to pursue the house Ground connection as the suspect. The latest electrical code in some areas actually requires that the house main Ground be established separately and you might need to go there if all else fails. But most homes are using the water supply line into the house.
That supply line only works because the pipe coming through the floor is actually the end of a long bare metal piece of pipe buried in the ground. It depends on contact of metal pipe surface with soil over a large area. If I read your post right, it was that buried pipe coming into the house from the street water main that was replaced. If the piece of new pipe was coated with something to prevent soil contact and make it last longer with reduced corrosion, or had an insulating wrap around it, or anything like that, it does not provide the contact area needed. On the other hand, if the piece of pipe coming up through the floor is actually joined onto the buried pipe under the ground with anything that does NOT preserve good electrical contact, you don't have the long length of buried pipe needed. So, check with the plumbing contractors for details of how that water supply pipe was installed.
You are right, though, to start with the connection to the new pipe. It is usually some simple two-piece metal saddle fitting that tightens onto the copper pipe with bolts. It then has a hole with a large setscrew to connect the Ground wire from your breaker box. If the fixture itself looks old and corroded and hard to clean, just buy a new one at any electrical supply or hardware store - easier that cleaning sometimes. But you might be able to clean the inner surface of the saddle fitting with emery cloth or steel wool or a wire brush. Similarly, use a piece of emery cloth to just polish the copper pipe in that short area before re-installing the saddle. Also look closely at the end of the copper wire, to be sure it is clean (and the inside of the saddle's hole) before re-assembling and tightening the setscrew. Don't forget, the setscrew and saddle clamp bolts do not need to be super-tight - they only need to be tight enough to bite into soft copper a bit. You don't want to puncture or squash the pipe!
When you investigate you might find an obvious dirty or loose Ground clamp on the pipe. But if you don't find and fix something obvious, consider having an electrician check that the quality of your household's master Ground connection is good enough.