before you scrap it or spend hundreds on an ecu, have you tried an autobox fluid and filter change? can work wonders when the box is playing up and is alot cheaper than just replacing parts in the hope that it cures it
Maybe you should get the fault codes read, maybe it's in "Limp Home" mode.
Re changing gear box filter and oil, i checked the oil on the dipstick and it looks very clean.
I will change the filter and oil, its got to be worth a go
If it was surely there should be a gearbox fault light showing?