You could also have you CEL(Check Engine Light) read for free at any Auto Zone. Good luck with this. I have no idea what exactly is causing this, but it doesn't sounds like a transmission problem honestly. Losing speed and shaking is usually caused by something with the engine or wheels, but I don't think it would be the tranny. I could be wrong though, but I would take it to another mechanic, or maybe there's a CEGer close enough that could come look at your car.
