Lexus LS Transmission Problems

That sounds familiar. It's like the transmission is wilting. Old fluid is a good suspect, but also think about undervoltage. Any other electrical issues? Maybe headlight dimming? A weak battery can mess with the transmission control unit.

The battery is new, but I'll check the voltage. If it's undervoltage, will a new battery fix it, or is there a bigger problem?

Think of the electrical system as a tree. The battery is the trunk. Undervoltage could be a parasitic draw in the branches. A bad alternator can also cause this. A new battery might hide the problem, but get a professional to find the real cause. Test the charging system with the transmission. A workshop visit is best.

Thanks for the advice. I went to a workshop, and it was undervoltage, like you thought. The repair, including fluid and the electrical issue, was 240€. The car runs great now.