intermitant overdrive

[heas1]

1998 astro awd
The overdrive works fine when the van is cold but as soon as the engine warms up and usually about the time the thermostat opens, the transmission changes down 1 gear and won't shift back into overdrive until starting from cold again
any ideas??

[old_master]

How many miles on the vehicle? When was the fluid and filter last changed?

[heas1]

hi
235k - I am having the fluid and filter changed this week - will keep posted.
thanks
c.