We need a little more information to help you.
1) What is your Slackware version?
2) What do you mean "by the default configuration for the 3.5 kernel"?
3) Did you download the kernel source from a kernel.org mirror?
4) Why do you need a 3.5 kernel? Does one of your hardware devices miss a driver in the provided Slackware kernels?
5) What is the purpose of the changes you made in the default .config?
6) How did you make these changes?
To compile a kernel for Slackware, read this how-to
from Alien Bob, one of the main Slackware contributors.
Other than that, to help us investigate you could append to your next post two .config files (one with which you could make a booting kernel a one with which there was a problem), respectively renamed as goodconfig.txt and badconfig.txt.
Also, outputs of "lspci -k" and "uname -a" would be useful.
PS To investigate yourself, if you think you miss some driver, look at output of "lspci -k" and check in the "bad" .config that all drivers/modules listed are present in this configuration. Or check that with the "search" feature of your configuration command ("/" for make menuconfig, Ctrl+F for make xconfig).