Anyone remember this science fiction story?

Quite a few years ago, I read this scifi story. One of the things that I y=though was rather interesting was the computers in the story, which were able to send problems back in time to themselves. This would allow them to have the answer when you asked, even if it took a long time to work on the problem. If the problem would take longer than they had been in existance, the computer would say ‘I have not yet been awwake long enough to anserw that question’.
I think that this story also involved some people from Earth, who were travelling in Earth’s first interstellar spaceship. After some distance, their ship stops after they cross some weird kind of border. It turns out that different rules apply in different parts of the universe, so once they cross this border, their fusion engine no longer works. They are rescued, and the people on the rescue ship say something like ‘Surely you didn’t expect the rules to be the same everywhere? Didn’t you bring an alternate mode of transportation, like dragons?’
That’s pretty much all I remember of this, but I would like to find out what it was! So, anyone else know of this?

I’m pretty sure you’re describing The Mantle of Kendis-Dai, by Margaret Weis and Tracy Hickman. It’s supposed to be a part of the Starshield series, although I haven’t found any sequels to it.