2003 Focus ZTS Engine Surge Under Load

I have a 2003 Ford Focus ZTS with a ZTec 4-cylinder motor, 5-speed manual transmission, @145K miles (one owner). I had the fuel pump replaced in October 2012. For the past couple of months I have had intermittent engine surges when the car is under load, and it is most prevalent when undergoing heavy acceleration or when going uphill. The engine idles smoothly at about 800-1000 rpm. The problem seems most prevalent when I have more than 1/2 tank of fuel in the vehicle. I have noticed that when the car is in cruise control mode and surges occur the accelerator pedal depresses as if the engine is trying to get more fuel. Spark plugs, Timing belt, tensioner, and water pump were changed approximately 30K miles ago. Alternator replaced @10K miles ago. No visible leaks around the head or valve covers. Vehicle is driven mostly highway miles in low dust environments. Engine light normally doesn’t come on, but today the surges were worse than normal and the engine light came on several times over approximately 1 minute interval. Then the surges stopped and the engine light went out and did not come back on.

Any suggestions?

By the way, when the problem started I changed when I purchased fuel to only buy fuel from a reliable fuel chain with a reputation for superior detergent fuel but the problem persisted. Have had oil changes and filters checked.