Nick-
I'm sure the gurus will chime in, however I'll give my two cents...
It does indeed sound like it is slipping. Your first bet would be to check the atx fluid. How does it look? It should look like a nice pinkish color, if it looking brownish, your fluid is bad. Also, take a whiff of the stick, if you have a strong metallic smell, this will further aid your prognosis.
Well, obviously since you haven't changed the fluid in 90k miles, it probably looks like crap. This is not a good thing. Your best bet would be to do the drain and fill method before getting a flush. Do it a few times and see if there is any improvement. After a while if the slipping stops, and you have completed a few intervals of the fill and drain method, you may go ahead and get a full system flush. However, I think when the tc starts slipping you have to take different measures, this is where one of the gurus will suggest your next course of action.
Good luck!
