Hi Mate, I have had an experience with a GQ diesel auto that had an auto failure and wad rebuilt. after which the transmission would drop out of lockup. After much testing it was found that the engine wad running too cool. The lock up in this transmission is sensitive to the temperature of the transmission oil. Ensure that the return oil line to the transmissoin is connected to the drivers side of the radiator and not bypassed by having an external oil cooler fitted.
We found the engine cooling system thermostat to be the cause of this.
Hope this helps.
Wads, GQ maverick Auto.