I have a 94 seville sls. it has about 147k miles and a 4t80e trans. It started to slip a little, I checked the fluid and added a half qt. After that, I had no gears. so I did a service as the fluid was slightly burnt. I had reverse for a few minutes if the rpm was at 1500. I now have nothing again. What could be the problem and how could I figure out what it is ? Thanks for your help.