Sias

My 02 ES 5spd has about 4k miles and recently started to surge real badly when the gas tank drops to 1/2 and below. It feels as if the car is going to run out of gas. Fill it up and everything is perfect, till it get's below 1/2 again! It does it intermittantly, so the dealership cannot find the problem... you all now the rest of that story, not a lot of help from them.
I did open the tank from the top, drained all the fuel out , cleaned and refilled with fresh gas, still the same? Also tried a few bottles of gas treatment. I don't think it is water in the system.
Any ideas??