2001 Taurus Surging

My '01 taurus has been surging on a daily basis lately. It usually happens when the engine is cold but it has also happened after the engine has warmed up, just not as often. It seems to shift out just fine, however, after it shifts into overdrive, it wants to surge twice, then it runs normal. The speed is around 40 to 50 mph when it does this. The check engine light has not been coming on so our code reader is useless. We thought it could be the egr valve so we changed it. It still surges. (Car seems to have a bit more power, though) I worry about transmission problems, but my boyfriend doesn't seem to think it's the cause. Transmission fluid color and level is all good. Any guesses on what's going on? Or should I just wait for my engine light to come on and then worry about it?

had a similar problem with my 01 today, anyone have an idea what causes this? Thank You in advance

I still don't have an exact answer, however, I did find something interesting while I was researching this. I'm not sure if this is my problem or not. Here's the link if you want to check it out.

http://www.taurusclub.com/forum/index.php?showtopic=17644&hl=surging

does this only happen at highway speeds? can you feel a miss if you power brake it?does it only happen when the trans shifts?
could be torque canverter, spark plugs/wires, fuel pressure low