Rough 3->4 shift in E53 4.8is (6speed) when cold

[FourPointEight-is]

I've got a 2005 that shifts quite rough when cold but ONLY for the 3->4 shift. All other gears are fine, as is reverse. Additionally, it seems fine once it warms up. I still can't believe how smoothly this thing shifts (except for that 3->4), smoothest auto I've ever driven.

Also, if I am REAL light on the throttle, it will shift smoothly. If you have more than 30% throttle it bangs into gear like someone gave you tap from behind.

The fluid & filter has been replaced (allegedly, I can't verify this information).

So knowing all this, what should be my port of call to start troubleshooting?

Apparently, I've read that the harness problem is only on the earlier models, and since the fluid and filter have been replaced already, I guess that leaves the shift solenoids as the next (relatively) inexpensive part to replace.

Advice?