1994 Pontiac Bonneville Am I being scammed?

In May 2010 I took my car into National Transmissions shop in Euless, TX and they rebuilt my transmission. They tore it completely apart and showed me the parts that were worn and needing to be replaced. They told me the price to rebuild it was $2700. They also determined I needed brakes including rotors at $501.00. I agreed to the repairs and they did the work. When I picked up my car, 2 days later, the transmission started slipping. I called National and they told me to bring the car back in, and I did. They kept it for almost 2 weeks and told me they tore it all back apart and it was ready to go, so I picked it up. I drove less than 2 miles and the tranny slipped and by the time I got to work 10 miles later, it had slipped 3 more times. I called National and they told me they drove it "all over everywhere" and it never slipped on them (it had the same gas in it as when I took it to them and they said they put no gas in it to drive it.). They told me to bring it in again, and the service writer John, drove it hard trying to get it to slip and it didn't but he wanted me to leave it over the weekend so they could drive it some more. The car was there 3 weeks and finally they called me and told me that my car was overheating and the heat from the radiator was causing the tranny fluid to break down and not allow the transmission to shift properly and I need to spend another $350+ to replace the radiator for the transmission to function properly. Is this true or are they just trying to get another repair out of me instead of dealing with the slipping transmission?
Thanks for your answer.