Was Troughton a bit doddery in his first story as well? I guess if every molecule of my body/personality had just been rewritten I'd be a little out of sorts too...
Did the 4th Doctor arrive pretty much fully formed personality wise? I can't recall...
The logic within the story might suggest that the character would be out of sorts . . . but as a viewer, I just want to see him in action!
Robot wasn't the best story but my recollection is that Tom Baker was pretty much in character from the get-go!