If you didn't play with the pole selection switch on the back of the tach. I would say you tach is done and needs replacing. If you tap the bezel of the gauge while the problem arises. Does the need sweep erratic. Or sweep to normal working order ?
It's rare you would need to change the regulator.
Is the fsm full of fuel ? There is a mechanical lift pump. And a float system. Have you checked to see if they are operational?
You can install another switch anywhere. You need to tap into the harness for the tilt and trim switch. RED is power 12v positive. BLUE is up wire. GREEN is down wire .
The Grey wire coming from the rectifier feeds the tach. Be careful, it is ignition powered, you may get a nasty shock if you touch any bare wire, while the engine is running. The purple wire is the 12v feed for the ignition system
ie the ignition switch. Radio feed , gauges etc.
Correct me if I'm wrong. But mercury had a service bulletin about the 25hp efi running issues. The correction was a new ground wire from the ecm ground wire to the starter bracket. The ecm ground wire is next to the starter, all the small ground wires are in that bunch.
Assuming the vent is working correctly on your fuel tank. It could be a lift pump issue. Not supplying enough fuel vst.
Now I do believe inside the vst tank, there is a float. Like in a carburetor. It meters the amount the lift pump supplies .
If all else fails. Start from scratch. Do a...
Could be your engine at one time overheated bad. Melting or distorting the water tube upper grommet. Your cooking system needs to be sealed to work properly. Now your cooling poppet valve or release valve could be open at idle, causing the water pressure to drop
Check fuse at voltage regulator , only if 1 exists. Otherwise check for battery voltage at red wire of voltage regulator engine off. Do you have a dva meter ? With a dva you can check high voltage output from stator while it's running. There are 2 or 3 yellow wires coming from the stator...