1999 Ford Windstar Transmission slipping

My Windstar has 144000 miles. Since 99000 the transmission has failed to shift into 3rd gear the first few minutes of travel or about 1/2 mile to one mile. After that it shifts fine for the duration of the trip. After cooling for a couple of hours (less in winter) it fails to shift again for the same distance. Recently another problem has surfaced. When accelerating on the interstate initially, the transmission shifts from OD to the lower gear and back a few times. I have had the oil and filter changed at about 135000 miles. I have also tried Lucas Transmission additive and it helped for a short time. My question is: Can I have the transmission overhauled successfully or should I buy a used one? Also if I wanted to attempt an overhaul myself, I can buy a Master kit for less than $300.00? I like my van! Thanks.