Vehicle is a 2003 Lexus GX470 with 309000 miles on it. While driving to the mountains it was fine on the highways I used the lower gears to slow myself down a bit as well as using the brakes going down the mountain then proceeded to use transmission normally in drive. Then going up through a town with slight sloping about 8,500 FT elevation car started to have issues or loss of power and although the speed limit was only 25 there it even struggled to do that I decided to turn back around. There was definitely a loss of power or something happening not being transmitted to the wheels. I will add there was no check engine light no oil temperature light no at temp light or any warning light at all. I also pulled over on level ground with the car heated up but the transmission oil dipstick and it was at the level it should be for hot operating temperatures.I was barely able to make it back down from the mountains home but I did. The next day when it was warmer outside and in town I drove it just a few miles and it seemed to drive normally like it had before the issue I even got on the highway and was able to get it up to 80 miles an hour. I didn't go far I just went a few miles and then came back. Anyone have any idea what the issue could be and potential things I should know? Thanks in advance.