1999 Subaru Legacy High Idle in

I've got a 1999 Subaru legacy that idles at about 2000 rpm. Its not consistent at 2000 rpm, but it consistently idles higher than 1000. Some days it idles at 1500 - 2000, sometimes it revs up to 3000 while I'm stopped at a light. The high idle is the main issue. Two (possibly related) but intermittent issues are a sudden loss of power followed by a jolting return of power on acceleration. This happens only at 2500 + rpm, and does not happen very often. Secondly, the engine sometimes revs quite high between gears when the clutch is depressed. When it does this, its not a brief rev before settling down - it may rev to 3000 or 4000 rpm and stay at that speed for 30 seconds or until I let the clutch back out.

I've cleaned the Idle Air control valve, and then replaced the Idle Air control valve and the throttle position sensor. The replacement parts were not new - I found a used throttle body on ebay that included both sensors. My hunch is that its unlikely both idle air control valves and throttle position sensors would be bad, but of course I could be unlucky.

The car is probably not going to last too many more years (rust in MN is pretty bad) so I'd like to avoid too many expensive sensor replacements. Any ideas about what tests I could perform to find the culprit?