Tranny slipping, advice needed

[jfnar]

I have an '05 2500HD that I used mostly for a hunting truck so it's not driven every day. Recently I've noticed that when it sits for a week without being driven the transmission will not pull for 5 to 10 seconds. After it gets started it's fine as long as it's being driven every couple of days but if it sits for a week it does the same thing all over again. I haven't had time to take it to a mechanic yet but I'd like some idea what's wrong before I do. Any information will be appreciated.

Jeff