99 Caravan Transmission slipping

Hi,

I have a 99 Caravan with about 300 000 km on it. I'm pretty lucky.
Besides the tower struts that were rusty and I needed to replace, I
have not had any major problems.

But, in the last couple of days my tranny has been acting up.

In the morning after staying outside in the cold all night the tranny
does not engage right away. This morning I put it in drive and instead
of going forward up the driveway the van started moving down hill. I
moved the gear selector around but the tranny would not engage. After
about a minute the tranny engaged and there were no problems the rest
of the day.

I have the tranny fluid changed every two years. It's due for another
change shortly.

Any ideas what could be the problem.

Thanks

Alex