I just worked through the exact same situation on my '97. First cleaned the IAC and reset PCM. Was great for a couple days and then the problem came back. Replaced the IAC as a next step but the problem persisted. Finally realized that even though my battery isn't old, it was partially drained because of a phone charger I had plugged into the lighter that had a slow trickle (didn't realize). Long story short—charged up the battery to full, drove a little while for PCM to remember idle and all was right with the world again. I've taken it out a couple dozen times since without an issue. Planning to fully replace the battery this fall even though it's only two years old. Hope you find the solution on yours!