I wouldn't worry about the tachs settling at a different point when you shut the engines off because it doen't really matter. To get to the tachs you need to remove the whole cluster they're attached to but it's simply a matter of removing a bunch of screws. The selecter switch on the back is probably the problem. As far as the shower sump I would first check it to make sure it's not the pump turning on and off which would mean the systems monitor is working correctly. You may want to make sure the check valve on the outlet of the shower sump is working because it could allow the water to flow back into the sump causing a short cycle situation.