Transmission slipping

I just had a Neutral Safety Switch replaced yesterday and when I left the garage the transmission would not shift beyond first gear while I was driving. I stopped and moved the gear shift to 3. The car ran fine and I made it home. I left about a 1/2 hour later and the same thing happened in drive only the service engine light came on this time. I was able to get it back to the mechanic since they are close by and they checked the installation and couldn't find anything wrong with that. The fluid is full, the the right color, no leaks are showing. These mechanics don't do internal transmission work so I need to find someone that does. I was trying to get a heads up on what the issue might before heading to the shop. As like with everyone else money is tight these days. Any help is greatly appreciated.