Bad idle

[Stanky Leg]

so my 91 turbo legacy idles really low sometimes, and we can't figure out why.
we changed the air filter, and it helped a little bit, but it still almost dies after letting it idle for maybe 10 seconds or more.
it hasn't actually died yet, but you can feel the whole car shaking, and the tach looks like it is down to less than 500 RPM.

any ideas?

[Legacy777]

Check for vacuum leaks.