[scottb]

The O2 sensor should be replaced every 60,000 miles. I'm not an expert by any stretch of the imagination, but my understanding is that it plays a part in fuel mixture. What you're describing sounds like a fuel problem, and if the sensor is going bad it's possible once it heats up it "rights" itself a bit and the car begins to perform normally. Since the manual calls for replacement at 60k, if you have that kind of mileage and haven't replaced it yet, it's worth doing.

On my 84, I had a problem where intermittently the car would fluctuate at idle between 800 and 1100, up and down, up and down. I replaced the idle control valve, and the problem went away.

Both of these are relatively easy and inexpensive "fixes," and present a good place to look before you begin looking at more expensive and complicated problems.