Don't START by spending the most money possible. You're gettin' hosed.
Check the condition of your wiring. The 96 & 97 had wire insulation issues that could be the cause of your problem. Since it started up in the rain that one time, I would also suspect a poor connection at one of the sensors - clean them babys out. Spray in some of that moisture protection stuff you can get at auto parts stores.
Because it has run fine for various considerable periods, and the symptoms were brought about by rain that one time, I would NOT suspect the need for rebuild or new PCM just yet.
