2003 Cadillac CTS

I have a problem that I back out of the garage and then go into drive and the car just barely moves forward. After moving very slowly it will start to move normally and after surging a little the transmission finally shifts normal than is fine the rest of the day. It only does this the first time in the morning. I took it to a transmission shop and they said they see what it is doing but that it doesnt do it long enough for them to find the problem. They could not find a code on the transmission or the engine. I bought a penn warrenty when I bought this used car but now Im afraid that something is going to happen after the warrenty because no one can find a problem with it.