My 1999 chevy astro van tranny is slipping

It is called an owners manual.

is there a check engine light on? There are numbers of things that can cause this.

Warning light means you have a problem with the air bag system.
Get it checked out ASAP.
Wouldn't want you driving around and have it go off.

Had the same issue with our 1999 chevy astro van, would crank but would not start. It had been raining and windshield had a leak, after 2 days of no rain it started. Pulled codes and was the Security system in the ignition switch. Hope this helps.

1999 CHEVROLET TRUCKS ASTRO VAN 4.3L 6-cyl Engine Code [W]
CAPACITIES: Automatic Transmission, 4L60-E Initial Fill..........4.7 liters
Automatic Transmission, Total Fill 4 speed 4L60-E..........10.6 liters
To check fluid level, transmission should be hot and engine running at idle. Pull the transmission dip stick out "red" wipe clean and re-insert. Check level of transmission fluid as indicated on the dip stick using the second reading and make sure the Astro is parked on leveled ground. Add Dexron III or VI as needed and do not over fill.
Thank you for using Fixya and good luck

You should have a 4l60E in your astro van. Which is electrically controlled, so no vacuum modulator. However, to address the ventilation selector not working... look for a vacuum line exiting the firewall and going to the passenger rear of the intake manifold. You will likely find it broken or disconnected there.

I have had that exact issue on my 99 astro,but before it left me stranded on the side of the Highway it was running a little rougher than usual,but once accelerated it was fine,until it finally Stalled. So i Replaced the fuel filter and The Cap and Rotor and now it is Fine,started Right Up.

Lock actuator on the rear hatch may be bad, I had to replace 3 actuators all at one on my van. I have the same problem with my drivers door you are having with your doors, today I'm gonna pull the door panel off and check the wiring and voltage.

sadly, it sounds like a bad torque converter.

im sorry to say no it will not.