2001 Grand Prix Transmission Problem

I checked the other posts as far back as my server would go & didn't find anything like this, so here goes.
I have a 2001 Pontiac Grand Prix, 3.1L V6, with about 75,000 miles on it.
(I bought it used w/ around 50,000 miles on it.) Lately, when I start it for the first time and put it in drive, it seems to slip in the transmission (RPM's go up) and then it "slams" into gear. It doesn't do this with any gear other than first, and it doesn't seem to happen any more during the day (yet). I don't know if the former owner ever changed the transmission fluid / filter. I'm planning to take it in for a few other fixes, so I'll get them to do that, too. Any ideas why it does this, and what to do to fix it?
Thanks.
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had the 4T60E transmission in it and it has the electric solenoids for shift control. Weird as it sounds mine did the same thing, bang once a day in first gear. Ended up being a bad ground on one of the transmission solenoids.
I drive for two years after that with no problems. Anyone else here of this?
Brian
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Thanks.
Where are these solenoids located? I asked a friend about it, and he had no idea what I was talking about.
Also, do you happen to remember about what they cost?

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Solenoids are under the transmission pan near the valve body I believe.
I don't know what they cost.
The technician that worked on mine said it was a loose ground on one of the solenoids. It just cost me labor, fluid and filter.
Brian

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