If power and MPG seem significantly weaker, you have no CEL, you've got good spark plugs and wires (if applicable) and a clean air filter, cleaning or replacing your upstream sensor may be a good move.
My dad had a '98 Dakota that ran but ran inconsistently; sometimes would just shut off at a red light. Long story short, I found one of the o2 sensors' connecting harness had melted against a portion of the exhaust pipe. No CEL though. Odd! New sensor and connector, truck ran like a champ. Major MPG improvement as well.