Transmission question

Hi. I have a 2002 Chevrolet Venture.
It has a 3.0L engine and has 40,000 miles on it.
Everything works well, except that if I use the car all day, by around late afternoon the tranny starts to shift abruptly, not terribly violent or slamming, but noticeably hard compared to when I just start using it in the morning or before lunchtime. Let me just make it clear, every morning until about lunch time, it shifts very smoothly as if it were brand new, then later in the afternoon (say 3pm onwards) it starts to shift abruptly. The only thing I could think of is maybe the tranny heats up later in the day (although it is pretty much Arizona weather all day), so I did a small test. I shut off the engine and let it cool for only 1 minute (exactly 60 seconds only), then I started the car up again. Voila! The abrupt shifting was gone. Of course, turning the engine off and letting it sit and cool for just a minute would not really bring down the engine temp much, and once started, it would heat back up to running temp (the temp it was in before I shut off the engine) in no time. So maybe it's not the temp, coz that small test did the trick. The abrupt shifting was gone. And stayed away for the rest of the day (I still drove the car for a couple of hours that day). However, the same thing happened again the following day, as it does everyday (like groundhog day). When it began to shift abruptly again late in the day or sometimes it happens early evening or at night when the weather is cooler. We've checked all the solenoids (all 4 of them) and they all seem to work (tested with a 12 volt supply), although one solenoid registered a "0" (zero) resistance with a volt meter, which makes you think it's always in the on-state or ON position. But there no warning or fault lights have appeared on the dash, so it's unlikely that any of the solenoids are busted. Now I'm thinking worn out clutch plates and clutch rebuild. Any ideas? Thanks.