which causes yum to keep 4 installed kernels around. You may want to wind this up to 10 or something. That way you will be more likely to have the older working kernel still installed after an upgrade.
Before you set this to a higher value, which is generally a good idea, you should first check how large your /boot partition is and work out how many you have space for. If /boot is just a directory in / then OK, but often /boot is a small partition on its own, and thus will only be able to accommodate a certain number of kernels...
Chris
