Hi guys... first post.
This morning I drove my 04 Malibu Maxx (3.5 V6 with 94,000 mi) all round here and there (drove approximately 25-30 miles). When I got near my home I noticed as I went up a hill near home (after stopping at a stop sign), the car shifted hard from 1st to 2nd... then not quite as hard (but harder than usual) from 2nd to 3rd. When I parked the car in the driveway... I just tested it by shifting from D to R... hard... then to P... back to R... hard.
Shut the car off... jacked it up and got under to see if anything looked wrong... nothing, no obvious fluid loss or anything apparent.
Started it back up after about 30 minutes. Shifts smooth as silk again.... drove around block, a mile or two, smooth.

Any ideas??

Without someone doing a diagnostic while the car is driven it will be hard to tell if this is a problem or not.

Have you checked the fluid? What about servicing the transmission? Fluid and filter change? Usually recommended around 50,000 miles.

Fluid checked out okay (level and condition). I have scheduled trans service at a local Chevy dealership. Advisor said that, while they do not change out the filter, they would do a pressure cleaning that would clean it, then refill with fluid. He said that was the standard trans service for this car. He said that the filter for this trans is inside the trans case, and cover would have to be removed for filter to be replaced. On another Chevy forum someone said they thought this problem was related to the shift solenoids and gave quite a gloomy picture.

The filter will eventually get clogged. If you have them "pressure clean" it you may not get what you actually want. Reversing the flow will knock some of the dirt off the filter into the bottom of the pan, but once you start driving again it'll eventually find its way back to the filter. Plus the reverse flow doesn't get all the dirt off, so it starts off with a reduced capacity.

When you change your motor oil you change your filter, right? When it's time to check your air filter and you find it's dirty, you replace it, right? (Unless it's a K&N, then you clean it.) Why just knock the tranny dirt off the filter into the tranny? They don't get all of the dirt out.

I favor replacing the filter. Yeah, it's a bit of a mess, but it needs done only once in a blue moon, not nearly as often as the motor oil. And it's the standard way it's been done for decades. They haven't decided to make a spin-on tranny filter and drain plug yet, so off comes the pan. I guess if it needed changed more often it'd have an external filter.

Take good care of your car and you stand a better chance of it taking good care of you.