Firstly, does the B&W boot with its original CPU? If so then you can narrow the problem down to Sonnet. Is the sonnet upgrade compatible with the B&W? Check their website.

Here's a good guide to different jumper settings on the B&W

http://www.dgweb.ca/g3/part5.html

If you have the Rev 1 motherboard that could also be a problem, those things were flaky in pretty much every way.