That makes it tough. Spitting fluid out of the vent hole is indicative of a plugged up trans. cooler so I bet the installer didn't flush it. Do you know if they even put in a new converter or just used the old one?
Slipping going into gear most likely indicates it was burned up. The rear clutch pack discs are a common failure part and when they go they put all kinds of tiny fragments into the cooler and lines, that's why they have to be flushed good. Those fragments also stick inside the converter and when you put in new fluid it loosens them up and they also go right into the new trans.
My guess would be the filter is plugged up not allowing enough fluid flow to keep the system charged by the time it's hitting high gear, but that's just a internet guess. It's possible it can be flushed good and that'll take care of the problem.
