long shift when car is cold

I am having this problem when I first start my car only. I can start my car and back out the driveway and shift to drive and drive several blocks before the first shift. Once is shifts the first time it works fine from then on. I can drive real slow or real fast (as fast as I can in 1st gear) and it will still wait a couple of blocks before shifting. If I park the car for over 6 hours I will have the same problem but if it is only parked a couple of hours no problem. It seems like there is someting in the transmission that needs to warm up before it will shift. I have tried to let the car idle for several minutes before starting out and it still waits before shifting. I have set and shifted from park to drive to reverse several times with the car running and waiting for it to go into each gear and it still waits to shift. The car is '91 300SE with 150,000 miles. Is it time to rebuild the tranny or is there something simple to try before I take it to a tranny shop. I have a name of a good shop that works on Mercedes from my mechanic so I am not going to take it to just any tranny shop.

Thanks,