1993 Chevy Astro slow response

I think your problem may be with the alternator/voltage regulator or the drive belt, isnt tight enough or needs replacement, try the drive/serpentine belt first, you may need a belt tensioner too, to keep the belt as tight as it needs to be, if the belt isnt turning the alternator as it should, your not gonna produce the amps needed to run the accessories properly, this may be why when you speed it up, the alternator will speed up too and then produce the necessary energy needed.

I just bought this van and it was inspected right before I got it. The serpentine belt was replaced as part of the inspection. As far as I know the alternator is original, so i'm assuming it's reached the end of it's useful life. Thanks

Agree, you can have it tested, for free, I think, even if its still in the vehicle. Autozone does this, I dont recall if you said the vehicle still drives.

https://www.2carpros.com/how_to/how_to_test_alternator.htm