car shifts wrong until trans gets hot

What would be the cause of a 240d transmission shifting hard in some gears and slipping into others until it is driven for approx. 5 miles on a warm day. After that, it shifts perfectly. I replaced all of the fluid and filter and triple checked the level at operating temperature. I recently added an aftermarket metal radiator. Is it possible that it is "overcooling" the fluid? I want to stress that the condition I describe is severe and not just standard warm up. It can take as long as two seconds to shift from 2nd to 3rd the first time through the gears even if I let the car sit and warm up to as hot as it will get before taking off the first time of the day. I thought at first this may be a vacuum problem but then why would the car shift perfectly after it heats up a bit? Any ideas? Thanks!