Harvey,
Watcha mate! It happens between all gears and at all temperatures but only so far under severe acceleration......when I'm having a bit of a play.
But generally I find my 'box has a few foibles. It hunts backwards and forwards between 3rd and 4th when doing about 80mph on motorways......which is kind of annoying.
Also, it sometimes changes between 1st and 2nd with a bit of a thump which is why I bought the Shift Kit from smallcar.com.
Basically, my tranny is a delight in all gears just not so good swapping between them.
As the 'box was rebuilt recently could it be more of a ECU problem, something somewhere sending bad messages to the transmission?