transmission slips first drive of the day

First off, LOVE the Magnum. It is a great Dad sports car... lots of room for the family but alot of power to drive like a sports car. Have had the reported problems with the front end and am currently replacing both wheel bearings in the front. That being said, I was looking to see if anyone else has had issue with the transmission on the first drive of the day. Had the transmission fluid and filter changed about a year ago, and in the morning the engine revs fine, but the transmission wont grab. If I use the mechanical slapstick to put it in first gear, it grabs fine, so it doesn't seem to be the pressure plate. I live in the northeast, and the problem is much more noticeable in cold weather. Any Thoughts?