|You fsb runs at 100Mhz default; agp bus is 66Mhz default, and pci bus is 33Mhz default.|
If your motherboard does not allow you to lock the agp and/or pci bus at 66/33Mhz or only offers you the otption of 112/37Mhz, you're gonna reach the OC limit pretty quick. In other words, the agp and pci bus speeds will increase as you increase the fsb.
The pci bus runs more stuff than just the pci slots. At 37 or 38Mhz you'll see audio, modem, nic, bsod or other screw ups.
See if you can take a look at cpu temps in bios.
Before you try to overclock further you need to identify the motherboard and find a way to monitor temps. MBM5 or SpeedFan are good programs for temp monitoring. CPU-Z may give you some information on your motherboard.