I've been trying to identify this novel for a while.
Ιn the story, AIs are able to travel backwards in time to ask themselves questions; therefore, older machines are able to answer increasingly difficult questions, having had more time to ponder the answer.
In the story, there's an ongoing revolt by thinking machines - the protagonists are looking for an ancient AI which will be able to answer the question of whether or not machines should serve humans.
I believe I read it back at a library back in the 90's - can't say for sure when it was published, though.