Bought april 2011 with 11000 miles

Bought april 2011 with 11000 miles. Automatic transmission slip in 2d gear (25 mi/hr) just once when cold. Began fall 2011 and all winter, once or twice always the first time at the biginning of a ride when cold, After goes ok. Never do it during all summer. Do it again this fall but 2 times in spite than one anf slip more. Last week, changed the valve body reconditionned, new selenoid and EPC(365$), New oil and filter.
Go perfect for 4 days. The fist day it was 0F degree outside in the morning ans did very well. After 4 days, began again the same thing, like last year. Just a little once or two.
What do you thing?