Does trans fluid wear out

My 02 ram quadcab has acted up twice this week. Its been over a hundred here and I let the truck run for 5 or 10 minutes when I run in a house to check it. When I come back and drop it in gear it seems to slip. I baby it and when I get up to speed it doesnt want to shift to o/d. I stopped both times at this point and let it sit an hour and it seems fine again. Does the fluid lose viscosity and make the converter loose.Changed it twice but havent done it lately,about 75 k ago was the last change. Truck has 160,000 miles. Could old fluid cause this in the heat or do I have a bigger problem looming.Long as I dont let it sit its fine. Whats gonna happen if i get in a traffic jam though. Rams have coolers on them already.What should I do?