'93 Bonneville, hesitates and now RPM's gone wild

1993 Pontiac Bonneville, 117,000 miles, V6 3.8 Liter

For a while now the car has to "warm up" regardless of the temperature outside before it will run right the first time it is started in the day. If you don't wait for it to warm up and try to go it will hesitate, and doesn't gain speed, will drive jerkily.

On the way home tonight, it was running fine until:
I was coming downhill and I hit about 55-60 mph and the car started jerking, and I noticed the RPM's going up and down like crazy. I was less than a mile from home so I kept going, and when I turned off the engine I noticed what was either a tiny bit of smoke or steam coming from under the hood.

Any ideas?