slackpkg isn't recommended to be used when upgrading kernels. You can see Pat's warning in /etc/slackpkg/blacklist
Code:
# Automated upgrade of kernel packages aren't a good idea (and you need to
# run "lilo" after upgrade). If you think the same, uncomment the lines
# below
The best way to upgrade kernels is actually to not update them at all. Instead, it is better to run installpkg against the different kernels you want to install and then you can have your old, known working kernel to fall back on if the new one doesn't work as intended.
When you use installpkg to install newer kernels, you will want to modify your /etc/lilo.conf to have an entry point to the older and newer kernels. I actually don't use the /boot/vmlinuz symlink at all in my lilo.conf, choosing to reference the direct kernel instead. This makes sure that I don't get caught off guard if something mistakenly updates that symlink (like installing a different kernel package).
However, going back to your original question, no, there isn't a way to specify which kernel you'd like installed short of editing your /etc/slackpkg/slackpkg.conf and moving the patches to a lower priority than %PKGMAIN, but that would downgrade all patches to their original versions, not just the kernel.
The easiest thing to do would be to manually download the original kernel-huge, kernel-generic, kernel-modules, kernel-source, and kernel-headers and run installpkg on them. Then you can edit your /etc/lilo.conf, run lilo, and see if the new (older) kernel works properly. Once you have verified the kernel you want is working as it should, you can removepkg the other kernel(s), being sure to specify the full path to it's log.
Code:
removepkg /var/log/packages/kernel*-3.2.83-*
