727 Sticks in 1st and
 

Hi,
I have a 440 mated to a 727, when the car has been warmed up and been used for at least 30mins then stops at lights in drive, you pull away but the gearbox stays in 1st on light/medium acceleration and doesn't change for 20s, then it will suddenly drop from 3500RPM and change into 2nd. After that it changes up and down fine.

It odd that this only happens when the car is warn, I have changed the fluid/filter once, any ideas where I should look next, anything I can check?

Thanks in advance!