Look in the 92-96fsm, in the 3.5 engine section, or in the mpi system, can't remember which right now. Steps to get at it are in several posts on here. Might as well get used to our cranky search function. Some search tips. It will search a 7 year range max, so set the search for from 7 years ago to today. Input the terms "butterfly shaft", and only select the mitsubishi tech section to search. It'll be in there, with a lot of extraneous stuff. Oh, yeah, search title and body of posts SECOND, only if a search of titles doesn't find your "lookin' for" in the titles.
It's way down in there, almost down to the top of the vee of the cylinder heads themselves. In short, you disconnect (and LABEL as you go, with pics, lots of pics, trust me...) all the little hoses and wire connectors and throttle cables and air hoses and probably the fuel lines (careful-a little fuel will squirt, catch rags needed), then remove the upper intake manifold, with the throttle body and plenum and intake runners (you'll see the two sets of intake runners, one set long and skinny for good flow velocity in the smaller tubes, and long length for good intake resonance ram effect all to boost low end torque, and a second, fatter, much shorter set of runners to feed the motor at above 3000rpms, fat for much higher air flow, and short for good ram effect and higher rpms.
Below this big old hunk of aluminum (which was bolted on with a couple of rows of long bolts - note and draw a diagram and sharpie #'s on the bolt heads so you put them back in the right place! - running down the centerline of the motor), there's the lower intake, bolted the the heads on each side, with a row of alternating size holes down the middle, the small ones mating to the low speed upper intake runners, and the large one mating the hi speed runners. Where these meet in the center of the lower intake, lives the possible gremlin, the intake runner changeover butterfly valves and shaft and what I strongly suspect are badly leaking shaft bushings, plastic doughnuts that are leaking air (high idle, lean mixtures, hard cold starting, bad emissions) and letting the butterflies flutter in the rushing intake air. This is MOST dangerous in the long run. The fluttering of the butterflies fatigues the tiny brass screws holding the b'flies to the shaft, and the round butterflies dive off into the cylinders and pistons and valves and combustion chambers and the flying pieces thereof soon engendered. Dead motor. $$$$$$$$$. You can cure it with a new lower intake with improved bushings, 400ish bucks, or bandaid it by unplugging the actuator hose and wiring the shaft arm in the location of your choice (normal rest postion is on the low speed runners, probably a good choice, I'd go halfway between as an experiment...) and sealing the outside air ends of the shaft with rtv goo
This is a doable if necessary task, so you check everything else in sight first. If your ecu is obd2, you can buy a cheap code reader and scan the codes and report back, and we can start sending you in the right checkin' directions.
'Twer me, I'd download the factory manual pdf from montymcv's site (I think the link is the the fag's) and read the stuff on the drivability troubleshooting probably in the fuel section, with error code lists and diagnostic trees that send you in reasoned out directions based on your described symptoms. Maybe you can do all the tests with home stuff, maybe not, but with the e codes and the book, we can logic most of it out from what you can dig up.
