Jay - If you get desperate, you might be able to create this problem on
the ground using a hand held or jury rigged electric motor to spin the
alternator rather than having to run the engine. This would allow
better diagnostic capability using an oscilloscope.

However, if the problem only occurs only at higher amperage loads, a
fractional hp electric motor won't be able to spin the alternator.
Likewise the RPM may be a problem. Alternators spin pretty fast at
cruise - like over 10,000 rpm.