You might want to hit the transmission forums for more in depth answers on this one but:
I've talked to several people that have noticed changes after a flush and fill. I didn't really notice any changes when I did mine, but I did notice changes after a few performance mods. If your "surge" is in fact the clutches engaging and locking up, at times it can feel like problems.
When I did my mods the 3rd gear lockup of the clutches was much more noticed, almost like a soft gear shift. I've also had times that the disengagement would take place with just a slight bit more throttle, when the engine was turning low revs. I guess that there can be a fine line between the lockup point and unlocking the clutches at times.
Are there any specific conditions your symptoms are taking place under?