I recently bought a 1985 500Sel. A very nice car, the previous owners put an engine in it with 42,000 miles on it. And they said they bought it and the transmission was bad and they had it rebuilt. They didn't drive it after they rebuilt it. It runs pretty good, but under hard acceleration it slips between 2nd and 3rd gear. This is what my grandpa told me after he test drove it. Since this is a rebuilt transmission what do you think the problem could be?