1993 Loyale Erratic Idle

[jetpilotdc10]

As the title says, my 93 Loyale has an erratic Idle.  Anywhere from 1100 to 2400 rpm. It will idle at a consistent speed for a minute or so, and then the idle speed changes.  On rare occasions it will do the 'ol rev, drop.....rev, drop....rev, drop.

This is my 1st Subaru so I'm not quite sure where to start with this one. My best guess is Throttle Position Sensor.  Any info on how to test it?

Any suggestions/shared experience is greatly appreciated!