Hello I'm hoping somebody may be able to help me out with an issue.

The transmission has recently started acting up. When the car is cold it will drive fine and shift at all speeds.. But after driving it for a little while it starts slipping at approximately 3rd gear and won't shift and over revs.

And then when I let off the gas and drive slower it's fine. I took it to a reputable mechanic and they did a diagnostic and they found no problems with it. They suggested it could be a problem where the car is going from electric to gas but it doesn't seem to run bad at all. Even tried changing the transmission fluid and made no difference.

Is this a known issue? I don't even know where to start. There isn't even any warning lights.