Hey, new poster here.
I just bought a 2005 Impala on Saturday. Was driving yesterday and the transmission started shifting hard in all gears. Got home and let the car sit for a while, then drove to the store. No shifting problems.
Drove it today and the trans started shifting hard again. Took to a mechanic, but the trans shifted fine for him. There was a P1811 code, but no check engine light. The car has under 98,000 miles. He cleared the code and said if it happens again to bring it back.
I have to do something soon as I only have a 30-day 1,000-mile warranty.
Any help would be greatly appreciated.

"I have to do something soon as I only have a 30-day 1,000-mile warranty."

Take it to a Chevy dealer. With a 1k warranty, you don't have options. Pay the dealer to diagnose and document the problem. A writeup from anyone else may not be enough to get the warranty endorsed.

Thanks, Guns.
The dealer will pay up to $1500, which would help out a great deal. A friend suggested clearing the adaptive shift memory from the PCM or having the dealer flash the PCM.
I agree the dealer may be the best option at this point.

my 03 monte did that a couple times, i had the trans fluid flushed and the new fluid put in and it was fine did it a couple times the rest of the time i had it. had that done at maybe 80k and 160 then sold it at 175k and never had to deal with it. my uncle bought the same and its the same just every once in a while feels like its stuck in the low temp shift mode.

You need at least pressure control solenoid if you have a P1811.