Feeling the heat

There is nothing that points to a problem with the Lantra auto in general that might cause your troubles. Given that the car is fine when cold -- when you would expect it to be getting extra fuel enrichment and perhaps some added spark -- and not when hot, I would have someone check the engine electronics for a fault. The only thing I would check on the auto would be the pulse generators. There are two, one on the input and one on the output. If they are faulty or dirty they can cause erratic shifting. Sometimes they will force the gearbox to hold a gear instead of shifting up or down. Starting off in a higher gear would feel like it has lost all performance the way you describe. The only thing to do is to take it back to a dealer and have it checked.