91 regal trans slips

I hope someone can help me diagnose the problem. I just had the trans flushed and filter replaced and now the trans slips like its in nuetral after you stop. I shift it to nuetral and back into gear, drive, then it will go. The car only does this when I stop at a stop sign or light. The car is has an automatic transmission, so I am not sure if its the torque converter or the modulator. I havent tackled trans problems like this before, so any help would be appreciated. Thanks.