2000 Sable bucking between 20-40 mph.

I have a 2000 Sable with a 3.0 liter 24 valve double overhead cam, with 106,000 miles, that all of a sudden started bucking between 20-40 mph, running cold or at temp. The check engine light has come on also. Thought maybe I had water in the gas on the last fuel fill up, so I filled it up at a different station and added Iso-Heat to the gas tank. Also made sure the gas cap is on tight. Changed the air filter, which was extremely filthy. Fresh PCV valve too. Car still bucks. Has great acceleration, and runs fine from 40mph and faster. Would one of the O2 sensors cause this all of a sudden problem? I was thinking about cleaning the EGR valve, but I'm not so sure this would all of a sudden cause this problem. I plan on taking it to have the codes read, just haven't had time yet. Any help would be greatly appreciated.

Update: After having the codes read, it came up as a PO401 code--The EGR valve has too little flow. So I pulled the EGR valve off and cleaned as well as possible with bristle type brushes. It was filthy. Test drove from cold to operating temperature, will little change. I then vacuum tested the EGR with the engine running at idle. When vacuum is applied the engine runs rough which means the EGR is operating normally. I then road tested the car with the EGR vacuum line disconnected and the vacuum line plugged, and the car runs great, with no stumble. So, my next try is the EGR Solenoid. Stay tuned.

After getting the code read and finding that it was an EGR related issue I did some investigative work on the internet and found that this model of Mercury was notorious for the DPFE sensor($80.00 at Ford) to go bad. They get moisture built up in them and slowly fail. I felt pretty confident that this was the problem and replaced it. The car runs very well now. No bucking or surging. The gas mileage should hopefully show some improvement too. I came across the bit about the DPFE sensor being a huge enough of a problem that Ford had a TSB out on it. It is TSB 04-11-01. It is titled: Driveability- Streamlined Diagnostic Procedure. Anyway the problem is fixed, all is good. Oh, one thing about the sensor for anyone experiencing similar problems. Even though Ford charges 2-3 times more than what the aftermarket part costs, and the Ford part is a direct replacement. I didn't have to change this, or convert that to get it to fit. Go with the Ford part. I'm sure it will last longer too. It only took 15 minutes to change the sensor.