I have the same problem, but am too lazy to do the repair. Likely your fluid is not cherry red, and may smell burnt. That means one or more clutch packs are burnt a bit, and that is a nasty repair. Luckly there are things you can do to still have a functioning tranny.
Change out the fluid in the bottom of the pan and the tranny filter, and do it regularly. ALWAYS USE ATF +3 or 4 (go with three if you can find it). NEVER EVER use dextron. Dextron has different shear characteristics and will burn/ruin your tranny. There are also habits that can make for a longer lasting first reverse clutch pack.
Transmission fluid is 0w20 for atf 3 I believe. When your transmission is cold, the fluid is more slippery ie the 0. Once it warms up, it becomes more grabby ie the 20. Let it warm up in park when it isn't too cold. Sadly, when in park, only some of the tranny fluid is being warmed. If you are able to drive for a bit and then it slips out of gear due to too much gas or trying to switch to second, you may want to warm it up in neutral with the parking brake on. This will take longer, but it warms up the fluid evenly. Find what works for you. I am in Canada, so the block heater is indispensable when it gets to minus 25 and below.
You may also try to park so that you don't switch gears from reverse to forward, and so that you are not going up a hill if you can avoid it. The gear switching exposes the clutch packs to fluid and makes it slippery. The hill means more torque and usually leads to slipping.
My van is 1996 3.8L Caravan
356000km <---- hard to justify taking out the tranny to replace the clutchpacks, especially since it drives well once warmed up.
Have a nice spring.