The '97 3.0L 1MZ-FE has DIS ignition, three coil packs on the front three cylinders (on top of plugs) with wires going from each of them to the rear. Unfortunately I don't think these plug wires are a replacement item...
How are we doing in terms of fuel? Toyotas have a "lifetime" fuel filter; they have a ridiculous amounts of filter element so the theory (keyword theory) goes that they will last the lifetime of the car. Check fuel pressure, pump, and so on. Usual suspects that you might check if a Bonneville were doing the same thing.
MAF sensor... rips in the intake tubing (in terms of running rough)... vacuum leaks... just some random ideas here.