I drove it to the store and went inside for about 20 minutes. I tried to start it and it just sat there chugging. I pumped the accelerator and it just kept chugging away, almost like a low rpm steam engine. Then it completely died and I couldn't get it to fire. The battery was strong but that was all.

It's a 97 F150 4.2 liter v6 w/101,000 miles. It's had a recent tune up and had the head gasket replaced last year.