Odd issue with transmission...

[Siegrisd]

I've searched on this quite a bit, but, really haven't found exactly these symptoms...

I have a 2001 E53 with the M62/5HP24 with 128k. It shifts wonderfully, there are no codes (scanned with a Foxwell NT642), the transmission has never gone into failsafe mode, however, while cruising in any gear, every 2.5 seconds or so, the engine RPM dips by about 150 RPM and then, almost immediately, comes back up to where it was. Initially, this didn't happen while in manual mode, however, now it does. So, it seems that the issue is getting progressively worse.

I've performed the adaptation reset (gas pedal to the floor, etc.), changed the fluid and filter (Redline fluid ATF-D6 with a MANN filter) and the issue still exists. The fluid was pretty dark and there was a light coating of black sludge on the bottom of the pan and a VERY minimal sheen of extremely fine brass particles. There were no chunks on the magnets, just a bit of thick, black goo. So, it seems like this would be pretty normal wear, but, by no means am I an expert in this stuff.

So, before I get totally wound for sound over ripping into things, there seem to be several possibilities to fix issue: solenoid wiring harness, TC going bad, main hydraulic solenoid going bad, clutch wear...

So, oh, transmission gurus, based upon the above, where should I go next and is there anything said here that could narrow this down?

Now... to start tearing into the timing chain rails and my rattling Vanos units... [sigh]