1996 Cadillac Deville Transmission Troubles

I bought a 1996 Cadillac Deville in early February. It had 105,000 miles on it and I gave it a good test drive and didn't find anything wrong with it. On April 1st I was leaving work and all of a sudden I noticed I couldn't easily get my speed above 35 MPH. I flipped on my driver information center and watched my RPMs and noticed that they were very high. I pulled to the side of the road and turned the car off and waited about 3 minutes then turned it back on and tried to drive again but was having the same problem. I pulled off the road into a parking lot and shifted my car into reverse and that worked fine. Then I shifted it into 1st and accelerated to ~20 MPH and tried to shift into 2nd gear but nothing happened. I tried 3rd gear also and nothing. Shortly after that I got a Check Transmission message in the driver information center. I limped my car home at 25 MPH and the next day took it to my local transmission shop. They took it for a ride and hooked it up to their computer and they told me my transmission was shot. I said go ahead and rebuild it. $1800 later, on this past Monday I got my car back and the car was shifting again into the other gears but I almost immediately noticed that it felt like it was slipping. I would be traveling at ~25 MPH and I would feel the car shift pretty hard into 2nd gear. I noticed the same thing at ~40 MPH and ~55 MPH. When I take off at a good clip I don't notice the shifting but when I'm traveling through town at a lower, steadier speed I definitely notice it. I'm wondering what is going on here, if the trans. Was rebuilt I shouldn't be having any problems at all with it should I? I'm beginning to wonder if they actually rebuilt my trans. Or just replaced a sensor or something like that and said they rebuilt the trans. And charged me nearly $2K. Can I get some thoughts/opinions and maybe some advice on what steps to take? I truly appreciate any help.