It has to be one of two things really. Its either an A-clutch problem or theres a problem with the 1st gear sprag (one way clutch).
If theres a hydraulic leak in (or to) the A-clutch then there may be insufficient flow from the pump to generate enough pressure to engage the clutch at tickover but the increase in flow produced from raising the engine revs (& hence oil pump speed) then overtakes the leakage, generates pressure & the clutch engages with a bang. The level of leakage is likely to be fluid viscosity dependent which is why I was asking if it varies with temperature.
The other option is the F-brake sprag. To test this you need to engage 1st gear manually which will override the sprag and engage the F-brake (this function is there to provide engine braking in 1st gear on demand). Would you be able to give this a try & see if it changes the speed at which 1st is engaged?
