I've been here.
Some dweeb updated things and made use of some table that only pcie has, breaking all agp cards in the process. Have a look at these threads
and the patch
I have kernel 18.104.22.168 there now on slackware-13.1 and that patch and all seems reasonable.
Have you KMS enabled? Another gotcha is that the firmware supplied by slackware is incomplete, and you need the 2 files referenced. KMS is achieved by
a. adding 'radeon.modeset=1' as a boot parameter
b. adding 'options radeon modeset=1' to /etc/modprobe.d/radeon.conf
c. Running an agp enabled kernel.
d. adding the firmware (referenced in the proronix thread) to /lib/firmware/radeon/
I'm running on the OSS driver, but I'm not a gamer