It sounds more like your kick down solenoid is sticking open, drop the pan and check the solenoid. You can get new ones on e-bay for about $40.00
This is straight from Holdenpaedia on oldholden.com
Drops Back to 2nd From 3rd and to 1st From 2nd Once Warmed Up:The Kickdown Solenoid is jammed open, is leaking or has failed causing continuously low Detent Pressure. Can be fixed by removing the Pan and replacing the Solenoid.
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