Have you confirmed mechanical timing? Is the harmonic balancer/crank pulley wobbling? If so, then the crank pulley bolt may have gotten loose (or was not tightened to spec during last timing belt job) and wollered out the woodruff key slot that aligns the timing gear with the crankshaft. This can cause weird runnability concerns some times (I'm dealing with this on a 2002 Montero Sport with a 6g72 right now). The hydraulic timing belt tensioner is also known to fail if run far past its service interval.
The work to fix it, depending on how much damage was done, is:
- get down to the crankshaft; similar amount of disassembly as a timing belt job, so this is a good time to think about a full timing kit refresh (also, crankshaft and camshaft seals)
- clean up the key slot; iirc Loctite 660 or other similar retaining compounds can take up to about 0.5mm of lost tolerance here (the Miata guys have a long, storied history with this repair; "Pine Hollow Auto Diagnostics" on Youtube repaired a Montero Sport with a wollered out key slot by mig-welding it, but he is a more courageous/skilled man than most)
- replace the woodruff key with a new one; original Mitsu is great, but the keys are a standard metric size that you should be able to find non-model specific options for (use caliper to figure out necessary dimensions)
- replace crankshaft pulley bolt and washer with new updated part (these were recalled, iirc your year also).
- reassemble everything:
- chase all threads (threaded holes and bolts, both), blow out all blind holes with compressed air
- use plenty of loctite (some people use high-strength red for the cranky bolt, but medium-strength blue is safer, since it doesn't need heat to be removed)
- follow all torque specs (quadruple check these), for the crankshaft pulley I'd suggest just getting the pulley holder tool (or weld up your own), the torque spec really matters here (i.e. don't use the strap wrench method).
Edit: My bad, your version of the 6g72 seems to use a manually-adjustable pulley-style tensioner (# MD104578). However, this poses the same woodruff key/cranky bolt concern.
PS: a wire thread about balancer wobble, cranky bolts, woodruff keys (and a cameo of a 91 3.0), good read:
/1076052/crank-pulley-wobble-with-new-pulley-and-tight-bolt-98-montero.html
