97 Grand Prix Recurring Tranny Probs

I have a 1997 Grand Prix GTP with about 190,000 Kilometers. (~110,000 Miles, or so).
About 6 months ago, the transmission began to slip badly. I had only
owned the car about 4 months at this point, so I attributed it to bad luck, perhaps a previous owner who abused it, and I layed out the cash and had it repaired.
The problem ended up being completely fried clutches - there was more clutch material in the pan vs. anywhere else.
So, I had it rebuilt by a reputable private shop.. It took a while for various reasons, but when I got it back I was mostly pleased.
There was an intermittent problem with the lockup, however, that I never really followed up on...most people probably wouldn't even have noticed it, but being technically adept I picked up on it ASAP.. Generally, it would only go into lockup when cold, and then not "firmly", but often with a slight shudder.
It almost seemed like it was somewhat "slipping" in lockup (Which didn't make much sense to me), and would rarely stay engaged for long unless I made a concious effort to keep it there...aka highway speed, *very* light accelerator pressure.
Anyhow, as most of my milage is city, it was of little concern - I was overjoyed with how the transmission was performing otherwise. Yes, I should have brought it back, but I couldn't do without my car for another 2 weeks. (Going back to the story above re delays...)
Anyhow, about 2 weeks ago, it began to slip again, as well as some other problems.
The slip is back, but more noticably now when a downshift is called for - the tranny seems to begin the downshift ok, but then never really seems to engage tightly into the lower gear, and if the accellerator is held, it will slip for a long period untill the load is removed from the engine, and then it will seem to "firm up" the shift, and you can continue as normal.
Additionally, with the cold weather upon us (REALLY cold recently), the transmission is balking at shifts when cold..I really have to back out of the power and give it an extended period of time to complete the shift.
Once warm, it seems better.
There is residue on the dipstick.
Anyhow...the gararge that did the repair the first time is going to stand behind it, and in early January it's going in for whatever needs to be done.
My concern is that there is an underlying problem that was not caught the first time around, and could come back again to haunt this second rebuild..
Anyone have any ideas? Was there a common problem with this year tranny?
Ideas?
Any info or suggestions appreciated, here or via email. (Remove the obvious spam-trap to reply via email).
Thanks Mark
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