Are you sure your Amprobe is one which is designed to measure D-C current - only a few of the Amprobe models are, while the others are for A-C current only. Some models are even further restricted to measuring at 60Hz only. The A-C models may produce a partial reading on D-C current which is not steady.

As the other poster Erik says, an accurate voltmeter can be of use since a good charging system will produce an overvoltage (14V or so) at sufficient engine revs (2000 r/m or so), and with a reasonable number of accessories running. It is probably not very useful to check the charging at idle as this will not force the alternator to produce its maximum possible output.

If the system can't produce around 14V or a little more, then the battery may be bad (such as an internal short demanding more charging current than even a good charging system can provide). The battery would then not hold charge for more than a few hours to a day if at all. It's also possible the charging system is weak (alternator not producing full output). Alternators I'm aware of produce 3-phase alternating current which is rectified by 6 internal diode rectifiers and all 3 phases are needed to contribute to a smooth D-C output. If one phase is missing (due to a bad diode or winding) the charging voltage will be low (more like 13.2 to 13.5 V) and will fluctuate at a rate proportional to engine rpm. The fluctuations would be too fast to appear on a voltmeter but would cause the meter to read low, since the voltage would be present only part of the time. A weak alternator may keep the car going under nice conditions (dry warm weather, highway speeds, daytime) but crap out under difficult conditions.

If you post again it may help to describe the symptoms from the beginning as well as what you've tried.