Re: checking for a draw of power
The tach gets its signal from the regulator/rectifier. One of the indications of a bad charging systems therefore is the fact that the tach doesn't work. The fact that its intermittent says you have some loose wires either on the tach, at the battery, or at one or more of the other connections. It could also be a heat related failure in the rectifier/regulator. If you indeed have a bad regulator/rectifier, one or more of the diodes may be the cause which is also allowing a path for battery voltage to ground.
I believe the tach gets its signal directly from the stator (it may be connected to the INPUT or AC side of the rec/reg, which is the stator wires), not the output of the rec/reg. (that's why tachs are switchable from 4 to 20 poles).
If your tach is working intermittently, it is likely that the charging portion of your stator is only working intermittently also.
If you measure directly across the AC terminals of the rec/reg with a voltmeter set to AC volts, you should see well above 17 Volts AC at reasonable RPM's (1500 or above). Measuring on the output side of the rec/reg, with the meter set to DC volts, you should see 13.5 or more VDC. You should see essentially the same (13.5 VDC or above) directly at the battery.
Silvertip is correct regarding the rectifier related diode short and its likely symptoms; but if your tach is working intermittently, i'd suspect a loose connection in the stator wiring common to both the tach and to the rectifier.
Check for a parasitic current draw, pull the positive battery cable, configure meter for DC Amps, 10 amps or more if possible, (you may have to plug your positive meter lead into a different meter hole to do this) leave all units that draw power turned off (ignition especially). Put one meter lead on battery positive terminal, other lead on battery positive cable. If you measure anything more than, say, 10-100 mA (.01 to .1 A), you've got a significant draw somewhere.
Good Luck.
