1994 Legacy - good power, stumbles on idle

[Zootal]

1994 Subaru Legacy. FWD. 4dr. Non-turbo. 230,000 miles or so. Normally purrs like a kitten.

Symptoms: engine will idle, but miss intermittently, every few seconds or so. Sometimes it's better, sometimes it's worse, sometimes it will stall and sometimes it won't idle at all. When it's acting up, it is hard to start. Has good power otherwise, no noticeable problems at higher engine speed. It's been doing this for weeks, but today it got so bad that it won't idle half the time and now my wife refuses to drive it (I don't blame her).

Here is where it gets interesting. With the key on and engine *off*, the tachometer jumps every second or so. It will jump up to 500-1000 rpm point, then drop to zero. The frequency of this is ever second or three, and is the same frequency that it stumbles at idle. The check engine light is *not* on.

This eliminates a whole slew of possibilities, and narrows it down to some weird electrical problem. It doesn't get worse when I hit a bump or slam the door (my old chevy vega would stall if I slammed the door, but that is another story).

Anyone have experience with this model and recognize these symptoms? I'm not sure what to do other than check the various electrical connections and think about replacing the ecu.

PS: The battery is fully charged and is fairly new, it cranks very well. Alternator is fairly new. It charges just fine when the engine runs. AFAIK there are no obvious mechanical problems, nothing makes noise, it normally just purrs and runs fine. Until this started happening.

With the engine OFF and key ON, the tachometer jumps, but the lights do not flicker, and no other lights or instruments act up.

Edited December 6, 2018 by Zootal