Hi,
Because the kernel isn't being loaded, the problem is probably not related to ACPI or APM.. but that also means it'll be hard to figure out why it's crashing like that.
My first impulse would be to check if the system has defective memory, via memtest86+ or similar.. my second would be to try using the i386 port of OpenBSD instead of amd64, you won't get the added benefits of a 64-bit operating system.. but you'll at least get a platform where you could offer developers access to try to find the cause of the hangup on amd64.
Very few developers hang out here, so you should
really file a bug report and/or post on the mailing lists detailing what you've tried and the symptoms of the bug.. this is pretty much all the advice we can offer you at the moment.
Mailing list archives:
http://marc.info/
Filing a bug report:
http://www.openbsd.org/report.html
Mailing lists & posting requirements:
http://www.openbsd.org/mail.html
Memtest86+:
http://www.memtest.org/
Hope that helps.