Hanging shift in 3rd on ZF 4HP22

[e34m30SF]

I want to say this is a smart transmission, but I am hoping it's not a tranny failure symptom. Every so often when I climb a slight grade at a moderate speed, the tranny seems to hang on to 3rd gear for a long while. Not sure why, not readily able to duplicate the problem. It still stays in 3rd even when i let off the gas and coast for a flat straightaway.

I'm usually going about 40 mph, 35% open throttle. I know Honda's transmissions have a grade-logic scheme, but does this wonderful machine have something similar too?

[BigKriss]

i'm pretty sure mine will go into 4th unless in sports mode

[JerseySi]

Mine ('88 535ia, same) only really shifts to 4th above 40mph on the flat, so up a gradient it will hold on to3rd if your speed is around the shift point

Has always done it, so I assume perfectly normal & nowt to worry about