Reboot when trying to load -current (amd64) kernel

Hi all,

I just tried upgrading from 5.4-STABLE/amd64 to 6.0-CURRENT/amd64, using
the conventional cvsup/make buildworld/make kernel method.  Everything
built cleanly, but upon rebooting, I found that all of the boot loader
options except "Escape to loader prompt" cause the machine to immediately
reboot with absolutely no indication as to the cause of the problem. 
Nothing is displayed on the console, even with the verbose option.  This
happens both with GENERIC and my own custom (no SMP/debugging) kernel
config.  However, as I mentioned earlier, I can escape to a loader prompt
and boot my 5.4 kernel with no problems.  The mailing list and
/usr/src/UPDATING don't seem to provide any hint as to what could be
wrong--is there something obvious I'm missing?

Here are the relevant lines from /etc/make.conf, in case that may have a
bearing on my situation:
CPUTYPE?=athlon64
CFLAGS= -O -pipe
COPTFLAGS= -O -pipe

Motherboard is Asus A8V Deluxe w/ Athlon 64 3500+.

Thanks,
Jason Harmening