Slipping automatic transmission?


True Classic
My daughter 2013 Fiat 500 with low mileage (less than 50k) started slipping when hot. Happen only sometimes after more thean 30 minutes drive. Carwon''t aceelerate anymore unless I let go the gas a few seconds then accelerate slowly. Doing that on all the gears. After it seems to be ok. But I could use the manual shifting and it works well. And then It's happening , the cruise control wouldn't accept to be set. My first idea is to check the automatic transmission fluid, but I'm afraid it could be the torque converter. I haven't done anything yet. My wife went to the garage, but when they drove it (only a few minutes) it was workig fine and if there no check engine light they won't want to take care of it. Changing the 500 transmission fluid seems to be involving (note the removal, but more the "filling" part....). Any tip?