ok, just hooked up the vacuum line to my boost guage, everything reads fine. only problem is sometimes when in neutral or clutch is engaged, i get a wierd idle (i think this might be the “bouncing tach” problem??) where it will go down to 700rpm then to 1300 rpm and back and forth. whats up?
vacuum leak maybe? did you tried disconnecting your boost gauge to see if the idle still bounces?
i have the same problem. but mine bounces from about 900 to 2000 rpm. i am desperate to find out what it is. please help! (i have already tried to search, but everything that people said did not fix it.)
Teggster, I am having a similar problem with my 93 LS, however it is just from 750-1100, yours sounds like one of my friends, it was his idle control sensor, or idle sensor. That is the problem I would think, but check around before you spend $$ on my advice 
ok here is your anwser guys…for my instance, i put a skunk2 intake manifold on my car. at first my car was jumping back and forth at idle also. my new manifold is quite larger than the old one so obviously more air comes in. the cpu is not used to this and has to adjust its self and the map sensor to compensate for all of the new air comin in. your problem will go away trust me. if you havent done anything to your manifold or havent recently gotten a turbo, check your vacuum lines around your map sensor. cause if you have not done any performance upgrades you have a vacuum leak…probably a disconnected hose(most common) or at worst your intake manifold gasket is bad
i had the same problem when i installed my turbo, it idles when stoped at a light, if it idles badly, fuels not burning enough or too much fuel is burning, your not going anywhere while stopped so all the fuel isnt burning, its drowning your engine, after you drive it, it goes away and repeat itself after stop.
about idleling back and forth, i still have the same problem sometime only, wiered! but i messed with my timing and now timing is somewhat, off after motor was taken all appart, i mean, all appart, intakemanifold, head, block and tranny was all seperated. now it idles, but i think you guys might be off timing or vacumm or try adjustiong your throttle.
idle back and forth is too much air comming in not going out enough from what i experience, i dont know how vacumm is going to cause it to go back and forth.
and vacuum leaks cant be stressed enough. can be very hard to detect visually. either you need to replace em all, use a vacuum/pressure gauge, dip em in water and apply pressure, or spray something on em like carb cleaner or starter fluid and see if the idle changes.
dont forget about the pcv and all the cleaning tips from the article above.