Seems to me that garbled BIOS comes from your AGP graphics card, which isn't happy at all at 133MHz FSB / 89MHz AGP, overheats and raves (if not plainly fried by now).

The GPU can be the Achilles' heel when overclocking 440BX boards to 133+. Some older cards can't just cut it, others need stronger cooling to go. As a rule one must implement overkill cooling before overclocking anything.

If the GPU works OK at 100 FSB (= it's still healthy), try applying quality fresh thermal paste under the heatsink and larger/stronger fan/s on it.

I've a V3 3500TV in there right now. Those get pretty hot even by default, I have a modern case fan blowing at it full speed. It still works fine, nothing fried there.

I saw you posting screens of a BX board overclocked @150Mhz in the other thread, which is quite a heftier OC, even with more cooling. Are you totally sure that my problem is just due to the AGP overclock? The thing that worries me most is that, like I said, it managed to delete the video card drivers and change some Windows settings by itself... Seems a bit hardcore for an OC that was common back in the day, doesn't it?

When all else fails, apply watercooling! 😁

Logically I can't be sure, it's just my best guess that the Voodoo AGP could be breaking your 133MHz party.

Found it, uncle Tom's March 2000 article on the subject:

Exploration into Overclocked AGP Graphics

Although the Voodoos passed uncle Tom's test at 133, they ran unhealthily hot:

I also had this topic concerning visual glitches in Descent and some other DOS games. Through testing I now found out that it's mainly affected by AGP bus speed rather than core clock; it seems to be still there at AGP@66, but much worse at 89, so this hints at the video card being the culprit as well. I'll do a new Windows install and see if any crap occurs without overclocking anything.

Another great article on the subject, from Anandtech's catacombs:

BX-133 Video Guide