You may have a malfunctioning electronic control unit (ECU), the main computer to the vehicle, this computer module can send faulty signals to your sensors causing the vehicle to perform erratically.
I really need more information to go off of, but I would make sure all scheduled maintenance is performed as well, i.e. spark plugs, fuel filter, etc.
Sounds like you could possibly have a transmission problem. I suggest you have it checked out as soon as possible. It could be a smaller problem now, but if you wait to have it repaired, it could turn into a much larger problem.
〉 Answered on Sep 4th, 2008 by Suzanne Grego, Technician at City of Philadelphia Fleet Management