I hope you check your voltage. This new K6 III runs at 2.0 volts, all other AMD chips runs at 2.2 volts. Which multiplier are you using and which memory speed are you using? It will run best on 100 mhz bus speed memory, so a multiplier of 4.5 X 100= 450 mhz or 5.0 X 100= 500 mhz. I do not think a bus speed of 66 mhz will help or be beneficial.
Also does the FIC VA-503 has the lastest bios? If my memory stands correct, you need a FIC VA-503+, not the A model, with 1.2 revision and the white sticker on motherboard with # E2568 or some # like that. If not you need to flash your bios (very dangerously) to upgrade your bios to support this chip.