I am guessing loose ground connection somewhere. Maybe the ground strap on the battery. Could be loose connection on the hot side also. Check your batter clamps also of course, but that is more obvious (although I have missed it before.) Very unlikely to have a bad Bosch regulator and about impossible to have 4 in a row. For a tester to diagnose "bad regulator" just means the voltage is wrong. If you have a loose or bad connection the voltage will be wrong, but only under load. So really it should say "loose connection or bad regulator."

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra