I have also been processing my colour films to but the results have been very inconsistant, I have noticed that the roll after processing varied from the start to the end, not drastically but there was a difference.
At first I put it down to my inexperience of C-41 processing but after reading up on the internet It seemed to point to the developing temperature varying slightly.
As I develop by hand and not with a processor I try to maintain the required 38 degrees C by placing the drum in warm water in-between inversions and maybe the outer frames of the film were at the proper temp while the inner frames were slightly under.
The Tetenal C-41 kit I use can also be used at 30 degrees C using different developing times which is a far easier temperature to maintain.
These shots are from the first roll of Fuji Superia 400 I tried using this method and from the first frame to the last the development was perfectly even.
click on an image to view larger……….